Add :special_wild_ignore option

This is a squashed version of this pull request:

  https://github.com/wincent/Command-T/pull/55

Specifically:

  https://github.com/myitcv/command-t/commit/754d91c597cc7666a8d458cc8f19e5dabb1d6dff
  https://github.com/myitcv/command-t/commit/3076cb1ca9d855b62e053e3a268e7200f8a957e8

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Drop "Special" from g:CommandTSpecialWildIgnore

As this is Command-T's only wildignore setting (ie. there is no
"non-special" variant), we can go with a shorter name here.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Update docs for g:CommandTWildIgnore setting

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Tidy up wildignore handling

The `#add_paths_for_directory` method was getting pretty long and ghastly,
so this commit removes the wildignore handling bits from that method and
moves them elsewhere.

Specifically, the shorter and simpler `#paths` method is now responsible
for setting the temporary wildignore setting and restoring the old
setting, if appropriate.

We now get the base wildignore setting only once, at initialize-time
(although you can force re-reading by doing a `:CommandTFlush`).

The logic for reading the wildignore setting has been moved into a
utility function on the CommandT::VIM module.

Overall, this is more verbose, but I think it makes the FileScanner code
cleaner overall.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Merge branch 'myitcv/special_wild_ignore' into next

* myitcv/special_wild_ignore:
  Tidy up wildignore handling
  Update docs for g:CommandTWildIgnore setting
  Drop "Special" from g:CommandTSpecialWildIgnore
  Add :special_wild_ignore option

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Fix a misalignment issue in the docs

Signed-off-by: Wincent Colaiuta <win@wincent.com>