Display links to editor in Differential and Diffusion

Summary:
It is possible to open a file in editor by registering a custom URI scheme
(pseudo-protocol). Some editors register it by default.
Having links to open the file in external editor is productivity booster
although it is a little bit harder to set up.
There are several other tools using file_link_format configuration directive
(XDebug, Symfony) to bind to this protocol.
I've added the example with editor: protocol which can be used as a proxy to
actual editor (used by Nette Framework:
http://wiki.nette.org/en/howto-editor-link).

Test Plan:
Configure Editor Link in User Preferences.
Register URI scheme in OS.
Open a file in Diffusion. Click on the Edit button.
Open a revision in Differential. Click on the Edit button.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1422