escape special chars in `<cword>` when run `:Ack<CR>`

[FuDesign2008]

In javascript / php, $ is valid in a variable.

function $sayHello () {}

If we run :Ack<CR> when cursor is under $sayHello, nothing will be found.

So try to escape $ and wrap <cword> in ' .

[benknoble]

I wonder if shellescape or fnameescape might help?

[FuDesign2008]

1. This commit is only fix for the word that includes $, is not a universal solution.
2. The source file has following comment. I'm not familiar with vimscript and can't find a better solution.

  " NOTE: we escape special chars, but not everything using shellescape to
  "       allow for passing arguments etc