Move Compiler instantiation to 'init' action

Move code for custom lessc implementations to the Plugin class to allow for filtering.

This also fixes a bug in 1.7.0 where the less.php and lessphp compiler paths included the github owner in the directory path ("lib/vendor/leafo/lessphp") when they are actually just at the project directory ("lib/vendor/lessphp").

Submitting as a pull request for code review. 
CC: @oncletom 
I would rather do a `public function getLessCompilerPath()` which returns only a string (something testable).

`instantiate_compiler` would simply do a `require $this->getLessCompilerPath();`.