restructure some code
new option: g:startify_skiplist_server

Before this we ignored all Vim server instances. That was a bad
decision. Apart from some weird cases, where servernames were set, it
totally slipped my attention, that gvim would always set a servername.

Sorry, for breaking this plugin for all gvim users!

I removed the check and added a configurable list of servernames to
be ignored.