Allow explicit relative paths even when includePaths set

[wabson]

Similar to #77 by @yoozze but uses an explicit mechanism to indicate when a path should be considered relative and not resolved using includePaths. In this PR the actual resolution is unchanged unless an include has a ./ prefix.

• Use leading dot (.) to indicate relative path
• Allow mixing of includePaths-relative includes and current-file-relative includes
• Allows recursive includes to work even when includePaths set (otherwise gulp options need to specify all recursively-included paths, which may not be known or difficult to specify)
• Added tests and README changes

[KenEucker]

This is the preferred solution, I think, and would knock out #77 on its way in.

Thanks for the tests! This will be merged in shortly.

[KenEucker]

@wabson this has been merged in, thanks for your contributions!

If you are still using this plugin and have the opportunity to test and verify that this feature is working as intended for you it would be awesome to get that confirmation!