Add scoop-alias command

Fixes #377

libexec/scoop-alias.ps1

@@ -0,0 +1,89 @@
+# Usage: scoop alias add|list|rm [<args>]
+# Summary: Manage scoop aliases
+# Help: Add, remove or list Scoop aliases
+#
+# Aliases are custom Scoop subcommands that can be created to make common tasks
+# easier. 
+#
+# To add an Alias:
+#     scoop alias add <name> <command> <description>
+#
+# e.g.:
+#     scoop alias add rm 'scoop uninstall $args[0]' "Uninstalls an app"
+
+param($opt, $name, $command, $description)
+
+. "$psscriptroot\..\lib\core.ps1"
+. "$psscriptroot\..\lib\help.ps1"
+. "$psscriptroot\..\lib\config.ps1"
+. "$psscriptroot\..\lib\install.ps1"
+
+$script:config_alias = "alias"
+
+function init_alias_config {
+  $aliases = get_config $script:config_alias
+  if(!$aliases) {
+    $aliases = @{}
+  }
+
+  $aliases
+}
+
+function add_alias($name, $command) {
+  if(!$command) {
+    abort "can't create an empty alias"
+  }
+
+  # get current aliases from config
+  $aliases = init_alias_config
+  if($aliases.containskey($name)) {
+    abort "alias $name already exists"
+  }
+
+  $alias_file = "scoop-$name"
+
+  # generate script
+  $shimdir = shimdir $false
+  $script = 
+@"
+# Summary: $description
+$command
+"@
+  $script | out-file "$shimdir\$alias_file.ps1" -encoding oem
+
+  # add alias to config
+  $aliases += @{ $name = $alias_file }
+  set_config $script:config_alias $aliases
+}
+
+function rm_alias($name) {
+  $aliases = get_config $script:config_alias
+
+  if($aliases.containskey($name)) {
+    "removing alias $name..."
+
+    rm_shim $aliases.get_item($name) (shimdir $false)
+
+    $aliases.remove($name)
+    set_config $script:config_alias $aliases
+  }
+  else { abort "alias $name doesn't exist" }
+}
+
+function list_aliases {
+  $aliases = @{}
+  (get_config $script:config_alias).getenumerator() |% { 
+    $summary = summary (gc (command_path $_.name) -raw)
+    if(!($summary)) { $summary = '' }
+    $aliases.add("$($_.name) ", $summary)
+  }
+
+  $aliases.getenumerator() | sort name | ft -hidetablehead -autosize -wrap
+}
+
+switch($opt) {
+  "add" { add_alias $name $command }
+  "rm" { rm_alias $name }
+  "list" { list_aliases }
+  default { my_usage; exit 1 }
+}