Updated syntax file to be ignore non-sense chars

Those in front of some commands (e.g. :set, :let, :highlight).

Thanks to filterfalse.
Highlight arguments of :execute command as exprs
Ignore colon and/or spaces before command names
Update syntax file to highlight arguments better

Much smarter then before.
Smarter highlight for :map and :command arguments
Merge branch 'better-syntax-highlighting'