changes to gulp-parse-toc.js to build on windows

[stombeur]

I wasn't able to build the output on windows 8.1 When loading the toc.md the path separators were windows-style, so the file path wasn't split correctly when using '/'. Next up when reading the .md files and linking to other files, the '/' was always used, but matched paths on disk were using the '\'

This now builds the pdf and the site at least. As I don't fully understand yet how files are pushed into the gulp-parse-toc.js plugin, I suppose this is not the best way to do it but I'm not sure how to fix it more elegantly.

changes

• toc file filePath split with path.sep instead of '/'
• replace path.sep with '/' for linked files

[michealbenedict]

Thanks @stombeur for testing this on Windows. This patch looks good to me. @sindresorhus @addyosmani comments? If this looks good, can we merge this in?