feat: config auto-generation

`cheat` now attempts to auto-generate a config file if one cannot be
found on the filesystem.

internal/config/paths.go

@@ -0,0 +1,42 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"path"
+
+	"github.com/mitchellh/go-homedir"
+)
+
+// Paths returns config file paths that are appropriate for the operating
+// system
+func Paths(sys string) ([]string, error) {
+
+	// if CHEAT_CONFIG_PATH is set, return it
+	if os.Getenv("CHEAT_CONFIG_PATH") != "" {
+
+		// expand ~
+		expanded, err := homedir.Expand(os.Getenv("CHEAT_CONFIG_PATH"))
+		if err != nil {
+			return []string{}, fmt.Errorf("failed to expand ~: %v", err)
+		}
+
+		return []string{expanded}, nil
+	}
+
+	switch sys {
+	case "darwin", "linux", "freebsd":
+		return []string{
+			path.Join(os.Getenv("XDG_CONFIG_HOME"), "/cheat/conf.yml"),
+			path.Join(os.Getenv("HOME"), ".config/cheat/conf.yml"),
+			path.Join(os.Getenv("HOME"), ".cheat/conf.yml"),
+		}, nil
+	case "windows":
+		return []string{
+			fmt.Sprintf("%s/cheat/conf.yml", os.Getenv("APPDATA")),
+			fmt.Sprintf("%s/cheat/conf.yml", os.Getenv("PROGRAMDATA")),
+		}, nil
+	default:
+		return []string{}, fmt.Errorf("unsupported os: %s", sys)
+	}
+}