Added experimental support for slinky_depends_external directive

This directive allows a file to depend on a file or files outside of
the manifest. Paths are interpretted as relative to the manifest root.
Globs are supported (implemented via Dir.glob). Changes are detected
via the mtime of the file.
Merge pull request #15 from mwylde/feature/depend_external

Added experimental support for slinky_depends_external directive