Adding ability to extend command line options with custom extensions

Here is my proposal to fix issue #1053

This will allow anyone to include custom command line extension for Compass. In order for a command line extension to work, it must be available as a gem that is prefixed with "compass-". Users can then include that in their projects by adding a command_extensions property to their config.rb file, such as:
```ruby
command_extensions = ["csslint"]
```
The above example would include the [compass-csslint](https://github.com/Comcast/compass-csslint) command line extension.

command_extensions is intended to take an array of strings, although I did also add support for it to be just a string if only 1 extension is desired to be included. The strings can include the "compass-" prefix as well:
```ruby
command_extensions = ["compass-csslint"]
```