generators: Teach StaticGenerator to walk FILES_TO_COPY

This allows you to copy directories (instead of just files) using
FILES_TO_COPY, and brings us into compliance with docs/settings.rst:

  `FILES_TO_COPY` (``()``)
  A list of files (or directories) to copy from the source...

For example, with and otherwise default configuration plus:

  FILES_TO_COPY = [('source', 'target')]

content/source/some-file will be copied to output/target/some-file.