Ability to define ignore list for symbol

I think it's nice feature to have ability to specify list of regexes to filter out symbols that should not be highlighted. For example there is no sense in selecting numbers or keywords. Just added defcustom highlight-symbol-ignore-list. 
Example - ignoring numbers:
```elisp
(highlight-symbol-ignore-list '("\\`[0-9]+f?F?[ulUL]*\\'"))
```