Add a formula for Arcanist - Phabricator's command line tool

Adds a formula to install Arcanist, the command line counterpart to
Phabricator. Phabricator is an open source code review (and more)
tool originally developed at Facebook and now used by a whole host
of companies e.g. Dropbox, Asana...

The formula wraps an existing script to clone the arcanist and libphutil
repositories to a local area on the system and warn if they are not
present on the PATH variable.