Syntax: Fasten highlighting, from 200ms to 20ms for first screen update

Problem:

The loading of a .md file was slow due to multiple nested lookback.
It is advised in vim help to prefer \zs, see :h /\@<
Anyway, it is useless to nest them.

089.080  000.085  000.085: sourcing /home/mtourneb/.vim/after/syntax/vimwiki.vim
089.264  005.184: opening buffers
089.322  000.058: BufEnter autocommands
089.323  000.001: editing files in windows
089.414  000.091: VimEnter autocommands
089.415  000.001: before starting main loop
303.698  214.283: first screen update
303.700  000.002: --- VIM STARTED ---

Solution:

Remove the function s:expand_delimiter in vars.vim.
It was doing a duplicate job with vimwiki#u#hi_expand_regex
And ... well avoid lookback as much as possible

091.784  000.087  000.087: sourcing /home/mtourneb/.vim/after/syntax/vimwiki.vim
091.991  006.034: opening buffers
092.050  000.059: BufEnter autocommands
092.052  000.002: editing files in windows
092.172  000.120: VimEnter autocommands
092.173  000.001: before starting main loop
112.574  020.401: first screen update
112.578  000.004: --- VIM STARTED ---

test/search.vader

@@ -7,10 +7,16 @@ Execute (Setup search testing wrapper):
       Log 'Cheating for old vim version, do not want to reverse bug'
       return
     endif
+
+    " Execute command and grab output
     redir => output
     silent execute a:search_command
     redir END
+
+    " The location list should not be empty
     Assert !empty(getloclist(0)), a:test_name.": no location list result"
+
+    " The location list should contains entries
     Assert match(output, '\d of \d') > -1, a:test_name.": no result message"
 
     " Tests that VimwikiSearch is quoting the pattern correctly.
@@ -30,7 +36,6 @@ Execute (Search test wiki):
   AssertEqual "= Test Wiki =", getline(1)
   AssertEqual "vimwiki", &filetype
 
-
   call TestSearch('VimwikiSearch foo',        'pattern with no spaces')
   call TestSearch('VimwikiSearch foo bar',    'pattern with spaces')
   call TestSearch('VimwikiSearch foo\bar',    'pattern with ''\''')