Implement a web backend for rustdoc_ng

This large commit implements and `html` output option for rustdoc_ng. The
executable has been altered to be invoked as "rustdoc_ng html <crate>" and
it will dump everything into the local "doc" directory. JSON can still be
generated by changing 'html' to 'json'.

This also fixes a number of bugs in rustdoc_ng relating to comment stripping,
along with some other various issues that I found along the way.

The `make doc` command has been altered to generate the new documentation into
the `doc/ng/$(CRATE)` directories.