Add list function to styler

Can list available themes, packages, and processors. Invoked by `styler
list` and then the intended target. When invoked without any valid
target will remind the user to supply one.

`list`, `--list`, `-l` are aliases and perform the same function.

Examples:

`styler list themes`

`styler --list packages`

`styler -l processors`

styler/.local/bin/styler

@@ -1,7 +1,6 @@
 #!/usr/bin/env bash
 
 readonly BASE_PATH="${STYLER_DATA_PATH:-${XDG_DATA_HOME:-$HOME/.local/share}/styler}"
-readonly DEBUG="${STYLER_ENABLE_DEBUG_MODE:-false}"
 
 readonly PACKAGE_PATH="$BASE_PATH/packages"
 readonly PROCESSOR_PATH="$BASE_PATH/processors"
@@ -14,6 +13,9 @@ main() {
   -s | --set | set)
     cmd="set_theme"
     ;;
+  -l | --list | list)
+    cmd="list"
+    ;;
   -v | --version | version)
     printf "Program theming script.\n\n©Marty Oehme\n\nVersion: 0.1.0\n"
     exit 0
@@ -75,6 +77,13 @@ get_processors() {
   done
 }
 
+# retrieves all installed themes from all packages, appends applications they exist for
+get_themes() {
+  local themes
+  themes=$(find "$PACKAGE_PATH" -type f -name 'base16-*')
+  echo "$themes" | sed "s/.*\\/base16-//" | sed "s/\\..*//" | sort | uniq
+}
+
 set_theme() {
   local theme="$1"
 
@@ -112,4 +121,23 @@ set_theme() {
   done
 }
 
+list() {
+  local selected="$1"
+
+  case "$selected" in
+  packages)
+    get_packages
+    ;;
+  processors)
+    get_processors
+    ;;
+  themes)
+    get_themes
+    ;;
+  *)
+    echo "Please select one of packages | processors | themes to list."
+    ;;
+  esac
+}
+
 main "$@"