Add an option to only highlight file that exists

[htfy96]

It seems difficult to improve the performance of this plugin in a short time. For example:

set guifont=CamingoCode\ 11

where CamingoCode\ is highlighted

let g:neocomplete#lock_buffer_name_pattern = '\*ku\*'

where ku\ is highlighted

"let g:neocomplete#sources#omni#input_patterns.php = '[^. \t]->\h\w*\|\h\w*::'

where input_patterns.php is highlighted.

These incorrect highlighted patterns really trouble me. Before these bugs are fixed(which I think might be quite difficult because there are so many special cases), I would rather choose to only highlight file that exists.

[htfy96]

Also, path including unicode characters is incorrectly highlighted.

[gu-fan]

yep, the file matching scheme is really difficult.

but check the existence of file takes more time, and will be async. havn't got a good idea how it work better