Rename githubissues plugin to enable functions

Simple prototype of omnicomplete and neo support
Omnicomplete works but is dang ugly
Tidy up omni code and make more universal

Update #6 Omnifunc completion / Unite source
Cache the issues data a bit

Update #6 Omnifunc completion / Unite source
Add option to disable omni
Update README

In correspondence to #6 Omnifunc completion / Unite source
Fix multiline issue