Forcing docs to use the cwd as file base

[kelsin]

This is a pull request meant to add to the discussion on #4

This is a pretty ugly hack

The only way I was able to get gulp-yuidoc to write docs with relative path names is to set the base on my files in this plugin. Doing it via gulp.src would cause weird behavior where I would have to check vinyl.base in lib/parser.js and pass vinyl.path or vinyl.relative and that felt like an even bigger hack.

This solution just sets the file base to vinyl.cwd and then passes the relative path into the filemap and dirmap.

Concerns: I'm very concerned that doing this will result in problems if people try to chain other commands in their pipes.

[coveralls]

Coverage increased (+0.32%) when pulling 7047159 on kelsin:relative-file-paths into b165187 on jsBoot:master.

[Robdel12]

+1 for this. I need this for one of my projects.

[josex2r]

+1

[nmcclay]

I'm kind of surprised this is still hanging out here. This PR works exactly as I would have imagined it would work by default (the same as yuidoc CLI). If the hack nature of this fix is the concern, perhaps it would be ok to simply give it an option flag in the gulp plugin?