Feature g1: Enterprise GitHub support and fixes.

Fixes #1 

* Add support for Enterprise GitHub. (Note some pretty egregious hacks here).
* Fixes incorrect error handling. Previously auth / API errors were eaten. The tool re-throws correctly now.
* Adds support for identifying `--state (open|closed)` of issues to report on.

/cc @rgerstenberger @per-nilsson @eastridge @alexlande