Auto-requiring compass command-line extensions & loading ad-hoc command-line extensions

As Chris Eppstein & I discussed at SassConf, this is a much simpler, more efficient, and more powerful solution than my [original proposal](https://github.com/chriseppstein/compass/pull/1261) to fix issue #1053 :-)

Assuming Compass command line extension are created with the naming convention of always being prefixed with "compass-", this will allow you to use that Compass command line extension by just installing the gem. No need for a command_extensions property in the config.rb, nor an extra "require" statement.

This also works with ad-hoc extensions as well (that's what line 17 is for)

If this is approved, then [compass-csslint](https://github.com/Comcast/compass-csslint) & [compass-csscss](https://github.com/Comcast/compass-csscss) will be much easier to use, and it will hopefully open the door up for others to create Compass command line extensions.