Add YASnippet metadata and placeholder snippets

This is partly done for parity with YASnippet, whose docs make a passing
mention of "snippets for writing snippets" (`vars`, `$f`, and `$m`). See
joaotavora/yasnippet-snippets@ac385b0.

Author: Alhadis

CHANGELOG.md

@@ -9,6 +9,7 @@ This project adheres to [Semantic Versioning](http://semver.org).
 [Unpublished]
 ------------------------------------------------------------------------
 * __Added:__ Comment highlighting for interpreter directives (hashbangs)
+* __Added:__ Snippets for [YASnippet][] metadata and placeholders
 * __Fixed:__ [Various inaccuracies][1] related to modeline matching
 
 [1]: https://github.com/github/linguist/pull/5271

snippets/snippets.cson

@@ -12,6 +12,22 @@
 			$3
 		"""
 
+	Snippet2:
+		prefix: "vars"
+		leftLabel: "yas"
+		description: "Extended snippet header with less common metadata."
+		descriptionMoreURL: "https://github.com/joaotavora/yasnippet-snippets/blob/ac385b0e/snippet-mode/vars"
+		body: """
+			# -*- mode: snippet -*-
+			# name: $1${2:
+			# key: ${3:trigger-key}}${4:
+			# keybinding: ${5:keybinding}}${6:
+			# expand-env: (${7:})}
+			# contributor: $6
+			# --
+			$0
+		"""
+	
 	Modeline1:
 		prefix: "mo"
 		body: "# -*- mode: snippet -*-"
@@ -40,9 +56,44 @@
 		description: "Optional human-readable label for snippet's menu entry."
 		descriptionMoreURL: "http://joaotavora.github.io/yasnippet/snippet-development.html#org592456a"
 
+	Field:
+		prefix: "$f"
+		body: "\\${${1:${2:n}:}$3${4:\\$(${5:lisp-fn})}\\}$0"
+		leftLabel: "yas"
+		description: "<code>${…}</code> field"
+		descriptionMoreURL: "https://github.com/joaotavora/yasnippet-snippets/blob/ac385b0e/snippet-mode/field"
+	
+	Group:
+		prefix: "g"
+		body: "# group: ${1:Menu grouping}"
+		leftLabel: "yas"
+		description: "Snippet menu grouping."
+		descriptionMoreURL: "http://joaotavora.github.io/yasnippet/snippet-development.html#orgd710184"
+	
+	Mirror:
+		prefix: "$m"
+		body: "\\${${2:n}:${4:\\$(${5:reflection-fn})}\\}$0"
+		leftLabel: "yas"
+		description: "`${n:$(…)}` mirror"
+		descriptionMoreURL: "https://github.com/joaotavora/yasnippet-snippets/blob/ac385b0e/snippet-mode/mirror"
+	
+	Author:
+		prefix: "au"
+		body: "# contributor: ${1:John Doe} <${2:john.doe@email}>$0"
+		leftLabel: "yas"
+		description: "Name of snippet's author or contributor."
+		descriptionMoreURL: "http://joaotavora.github.io/yasnippet/snippet-development.html#orge2b38b9"
+
+	AtomDescriptionMoreURL:
+		prefix: "url"
+		body: "# atom-description-more-url: ${1:http://more.info/}"
+		leftLabel: "yas"
+		description: "Atom-specific header used by `yas2atom`"
+		descriptionMoreURL: "https://github.com/atom/snippets#optional-parameters"
+	
 	AtomSelector:
 		prefix: "as"
 		body: "# atom-selector: ${1::not(*)}"
 		leftLabel: "yas"
-		description: "Atom-specific header used by <code>yas2atom</code>."
+		description: "Atom-specific header used by `yas2atom`."
 		descriptionMoreURL: "https://github.com/Alhadis/YASR"