Add support for a CommandT-local wildignore

Via the variable `g:CommandTSpecialWildIgnore` you can now set a CommandT-local `wildignore`.

This allows extra paths/file extensions to be excluded from the CommandT scan, but for those paths/file extensions to remain via `:sp`, `:e` etc

```vim
set wildignore=*.o,*.class,*.so,*.so.*,*.a,*.tar,*.tgz,*.gz,*.tbz2,*.rpm,*.zip,*.ZIP,*.gif,*.GIF,*.jpeg,*.jpg,*.JPEG,*.JPG,*.mp3,*.MP3,*.mpg,*.MPG,*.avi,*.AVI,*.asf,*.ASF,*.ogg,*.OGG,*.aux,*.log,*.bbl,*.blg,*.brf,*.cb,*.ind,*.idx,*.ilg,*.inx,*.ps,*.dvi,*.pdf,*.toc,*.bak,*.png
let g:CommandTSpecialWildIgnore=&wildignore.",**/bower_components/*,**/node_modules/*"
```

All tests pass with this commit