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.

doc/startify.txt

@@ -118,6 +118,16 @@ Example:   let g:startify_skiplist = [
              \ ]
 
 
+============-
+
+    let g:startify_skiplist_server = []
+
+Do not create the startify buffer, if this is a Vim server instance with a
+name contained in this list.
+
+Example:   let g:startify_skiplist_server = [ 'GVIM' ]
+
+
 ============-
 
     let g:startify_unlisted_buffer = 1