Add :dependencies option, for checking changes an assets

If your asset files include other files (e.g. sass's @import command),
you don't just care about changes in the main asset file, but all
files that the asset depends on.  This option makes it possible to
do things like:

  :dependencies=>{'assets/bootstrap.scss'=>Dir['assets/bootstrap/**/*.scss']}

Any when any of the bootstrap .scss files are modified, Roda will
treat the asset as modified instead of using a 304 response.
Remove :cache option, unused

The render option has a separate cache, but this :cache option did
not affect that.