Make main_file to be relative to the less directory, rather than the current directory

This is just a simple change to fix issue #58, and some of the issues that may be in #60

The main benefit of using the less directory rather than the current directory is so .less files in subfolders can trigger an update for the main_file.

and like kimsey0 said in his pull request please forgive me for not following any conventions regarding pull requests. This is also my first one, and I didn't know if there were anything I should have done before submitting it.