Massive theme overhaul (#62) ...

* progress on overhaul

* finish scrutinization of languages existing in old colors file

* remove markdown code block highlight due to broken upstream syntax

* add user config globals

* update issue and pull request templates

* small fixes

* fix "illegal value" error when using overrides

* actually fix the attribute filter this time

* add git commit diff colors. closes #46

* remove unnecessary duplicate

* set SignColumn explicitly

* Add a helpfile (#64)

* Ignore the tags file generated by :helptags

* Write the help file

Added README and LICENSE