Add a mechanism to skip copying static files from a parent template

[hegemonic]

I'm working on changes that will make it possible to create child templates. As part of those changes, I'm setting things up so that if a child template and a parent template both specify static files to copy, Baseline will merge the lists of static files and copy all of them. This creates problems if, for example, a child template overrides Baseline's default fonts; in this case, there's no reason to copy Baseline's font files to the output directory.

There should be a config setting that allows child templates to prevent specific static files (or directories of static files) from being copied to the output directory.