Implement a use_warnings flag for the Mason compiler

This allows you to catch runtime warnings in all Mason templates by
enabling a global use_warnings switch, similar to the use_strict
switch. Unlike use_strict this is off by default, so it's fully
backwards compatible.

I have a huge codebase where I'd like to enable warnings by default, and
not have to turn them on one-by-one for each template, which would
be *really* painful.
Merge pull request #4 from avar/implement-use_warnings-switch

Implement use warnings switch