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

README.md

@@ -51,7 +51,9 @@ Installing
 Configuring
 -----------
 ### conf.yml ###
-`cheat` is configured by a YAML file that can be generated with `cheat --init`:
+`cheat` is configured by a YAML file that will be auto-generated on first run.
+Should you need to create a config file manually, you can do
+so via:
 
 ```sh
 mkdir -p ~/.config/cheat && cheat --init > ~/.config/cheat/conf.yml
@@ -89,7 +91,8 @@ const squares = [1, 2, 3, 4].map(x => x * x);
 ```
 
 The `cheat` executable includes no cheatsheets, but [community-sourced
-cheatsheets are available][cheatsheets].
+cheatsheets are available][cheatsheets]. You will be asked if you would like to
+install the community-sourced cheatsheets the first time you run `cheat`.
 
 
 Cheatpaths