feat(installer): implement "installer"

Squashed commit of the following:

commit 5c322e79b7
Author: Chris Lane <chris@chris-allen-lane.com>
Date:   Fri Mar 6 19:56:56 2020 -0500

    docs(README): update the `README`

    Update the `README` to document the improved config-generation
    mechanism.

commit 803e1f014c
Author: Chris Lane <chris@chris-allen-lane.com>
Date:   Fri Mar 6 19:19:49 2020 -0500

    feat(config-init): platform-specific pathing

    Update `--init` subcommand to rely upon the same platform-detection
    intelligence that was previously implemented by the "installer".

    The installer and `--init` should now produce identical config files.

commit 99c48097e2
Author: Chris Lane <chris@chris-allen-lane.com>
Date:   Fri Mar 6 18:26:33 2020 -0500

    feat(installer): platform-correct config templating

    Modify the "installer" to populate cheatpaths with sensible defaults
    based on the detection of the user's operating system and environment.

commit 8e1580ff5a
Author: Chris Lane <chris@chris-allen-lane.com>
Date:   Thu Mar 5 20:19:58 2020 -0500

    fix(tests): fix `config.Paths` tests

    Refactor `config.Paths` (by externalizing a call to `homedir.Dir`) to
    decouple it from filesystem paths, thus facilitating cleaner unit-tests.

commit a08dca70d9
Author: Chris Lane <chris@chris-allen-lane.com>
Date:   Thu Mar 5 18:14:27 2020 -0500

    feat(installer): default path selection

    Modify the installer to improve default config and cheatsheet path
    selection.

commit e15bc6c966
Author: Chris Lane <chris@chris-allen-lane.com>
Date:   Thu Mar 5 17:49:50 2020 -0500

    fix(typo): correct comment typo in `main.go`

commit efd09575df
Author: Chris Lane <chris@chris-allen-lane.com>
Date:   Thu Mar 5 17:46:49 2020 -0500

    feat(config): refactor config path detection

    Previously, failing other checks, on Unix and BSD systems,
    `config.Paths` would attempt to compute the user's home directory by
    reading the `HOME` environment variable.

    This change deprecates that approach with a call to `homedir.Dir`, which
    is used elsewhere throughout the application.

commit ec10244ebe
Author: Chris Lane <chris@chris-allen-lane.com>
Date:   Thu Mar 5 17:15:28 2020 -0500

    chore(installer): delete unused file

    Delete `installer/installer.go`, which (in hindsight) was unnecessary.

commit ebd9ec6287
Author: Chris Lane <chris@chris-allen-lane.com>
Date:   Wed Mar 4 19:31:13 2020 -0500

    wip(installer): stub experimental "installer"

    Stubs out an experimental "installer" that will help new users to
    quickly configure `cheat`.

commit ecac5a0971
Author: Chris Lane <chris@chris-allen-lane.com>
Date:   Wed Mar 4 19:30:12 2020 -0500

    chore(dependencies): updates vendored dependencies

internal/installer/clone.go

@@ -0,0 +1,24 @@
+package installer
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+const cloneURL = "https://github.com/cheat/cheatsheets.git"
+
+// Clone clones the community cheatsheets
+func Clone(path string) error {
+
+	// perform the clone in a shell
+	cmd := exec.Command("git", "clone", cloneURL, path)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	err := cmd.Run()
+	if err != nil {
+		return fmt.Errorf("failed to clone cheatsheets: %v", err)
+	}
+
+	return nil
+}

internal/installer/prompt.go

@@ -0,0 +1,37 @@
+package installer
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+// Prompt prompts the user for a answer
+func Prompt(prompt string, def bool) (bool, error) {
+
+	// initialize a line reader
+	reader := bufio.NewReader(os.Stdin)
+
+	// display the prompt
+	fmt.Print(fmt.Sprintf("%s: ", prompt))
+
+	// read the answer
+	ans, err := reader.ReadString('\n')
+	if err != nil {
+		return false, fmt.Errorf("failed to parse input: %v", err)
+	}
+
+	// normalize the answer
+	ans = strings.ToLower(strings.TrimRight(ans, "\n"))
+
+	// return the appropriate response
+	switch ans {
+	case "y":
+		return true, nil
+	case "":
+		return def, nil
+	default:
+		return false, nil
+	}
+}