Add a syntax highlight dropdown, if pygments is enabled.

Summary:
- Add a default list of supported languages to default.conf.php
  and make the initial/default value customizable.
- Store a '' in the database to infer the language from the filename/title.

Test Plan:
Tested in my sandbox with pygments enabled and disabled and various
combinations of filename/extension/dropdown selection.

Reviewers:
epriestley

CC:

Differential Revision: 587

src/applications/paste/controller/view/PhabricatorPasteViewController.php

@@ -96,7 +96,7 @@ class PhabricatorPasteViewController extends PhabricatorPasteController {
 
     $text_list = explode(
       "\n", $highlightEngine->highlightSource(
-        $paste->getTitle(),
+        nonempty($paste->getLanguage(), $paste->getTitle()),
         $file->loadFileData()));
 
     $rows = $this->buildDisplayRows($text_list);