Add the ability to remove source media files when building extensions.

When packaging extensions, it's now possible to strip away all source
media files (non-minified JavaScript/CSS/LessCSS files) from the
resulting package. These are generally not needed, and doing so can
reduce the size of the package.

There's a 'remove-source-files' option that can be set in setup.cfg that
will cause all JavaScript/CSS/LessCSS source files to be removed from
the build. This can be set by adding the following to setup.cfg:

    [build_static_files]
    remove_source_files = 1

Reviewed at https://reviews.reviewboard.org/r/5775/