Parse content from a stream.

Here is the initial code for parsing content from a stream.

When writing tests I find it much easier (and faster) pass StringIO
instead of writing content on a real disk.

This branch add initial support for that.

I have updated LessLexer.input() to allow both a stream and a path, since yacc API is ugly.

I have not updated LexxLexer.file() method since I have no idea how and where it is used.