Add support for API Blueprint

This adds a grammar and samples for [API Blueprint][] based on the
[Sublime Text plugin][] from Apiary. The [Atom language][] is also based
on the Sublime plugin.

API Blueprint is now used in over [3,600 repositories][] on GitHub and has
[several][Aglio] [open source][Dredd] [tools][Drakov] available.

Here's an [example using Lightshow][Lightshow] to highlight a small API.

[3,600 repositories]: https://github.com/search?utf8=%E2%9C%93&q=FORMAT+1A+extension%3Aapib+extension%3Amd&type=Code&ref=advsearch&l=
[Aglio]: https://github.com/danielgtaylor/aglio
[API Blueprint]: https://apiblueprint.org/
[Drakov]: https://www.npmjs.com/package/drakov
[Atom language]: https://github.com/danielgtaylor/atom-language-api-blueprint
[Dredd]: https://github.com/apiaryio/dredd
[Lightshow]: https://github-lightshow.herokuapp.com/?utf8=%E2%9C%93&scope=from-url&grammar_url=https%3A%2F%2Fgithub.com%2Fapiaryio%2Fapi-blueprint-sublime-plugin%2Fblob%2Fmaster%2FAPIBlueprint.tmLanguage&grammar_text=&code_source=from-url&code_url=https%3A%2F%2Fraw.githubusercontent.com%2Fapiaryio%2Fapi-blueprint%2Fmaster%2Fexamples%2F12.%2520Advanced%2520Action.md&code=
[Sublime Text plugin]: https://github.com/apiaryio/api-blueprint-sublime-plugin

samples/API Blueprint/actions.apib

@@ -0,0 +1,55 @@
+FORMAT: 1A
+
+# Advanced Action API
+A resource action is – in fact – a state transition. This API example demonstrates an action - state transition - to another resource.
+
+## API Blueprint
++ [Previous: Resource Model](11.%20Resource%20Model.md)
++ [This: Raw API Blueprint](https://raw.github.com/apiaryio/api-blueprint/master/examples/11.%20Advanced%20Action.md)
+
+# Tasks [/tasks/tasks{?status,priority}]
+
++ Parameters
+    + status (string)
+    + priority (number)
+
+## List All Tasks [GET]
+
++ Response 200 (application/json)
+
+        [
+            {
+                "id": 123,
+                "name": "Exercise in gym",
+                "done": false,
+                "type": "task"
+            },
+            {
+                "id": 124,
+                "name": "Shop for groceries",
+                "done": true,
+                "type": "task"
+            }
+        ]
+
+## Retrieve Task [GET /task/{id}]
+This is a state transition to another resource
+
++ Parameters
+    + id (string)
+
++ Response 200 (application/json)
+
+        {
+            "id": 123,
+            "name": "Go to gym",
+            "done": false,
+            "type": "task"
+        }
+
+## Delete Task [DELETE /task/{id}]
+
++ Parameters
+    + id (string)
+
++ Response 204