swc-windows-installer.sh: Install nano source syntax highlighters

Make writing Python source code (and a number of other languages) a
bit more exciting for the eyes and easier on the brain by coloring
strings, comments, and such.

We need to install the source tarball because the compiled zip doesn't
include the syntax highlighting scripts.  To make that easier, I've
extracted out the download / hash-check functionality into a new
download(), which I use in both the old zip_install() and the new
tar_install().  If the user doesn't have an existing ~/.nanorc, we
populate it by adding 'include' options for each of the syntax files
in the Nano tarball.  If they do have an existing ~/.nanorc, I assume
they know what they're doing ;).
Merge branch 'master' into bc-namespaced

* master:
  swc-windows-installer.sh: Install nano source syntax highlighters