Configure LESS options from an external .lessrc file

I needed a simple way to configure the LESS compiler for my application, so I added support for declaring options in a .lessrc file in the project directory. Inspired by @jonschlinkert's implementation in [assemble-less](https://github.com/assemble/assemble-less#lessrc).

I use the config file to predefine additional paths for my imports and enable compression, etc. This could also be useful for enabling/disabling source maps once the dependency gets bumped to the new version (#1565).