Expand '~' in the skiplist

The skiplist gets checked against normal Vim regular expressions. That also
means that you can't just use '~'; see ":h /\~".

That especially sneaky if you use variables that refer to directories within
your home directory.

Solution: expand them via fnamemodify().

References #154.

autoload/startify.vim

@@ -20,7 +20,7 @@ let s:session_dir    = resolve(expand(get(g:, 'startify_session_dir',
 
 let s:skiplist = get(g:, 'startify_skiplist', [
       \ 'COMMIT_EDITMSG',
-      \ $VIMRUNTIME .'/doc',
+      \ fnamemodify($VIMRUNTIME, ':p') .'/doc',
       \ 'bundle/.*/doc',
       \ ])