Add hardLink option to grunt.file.copy

I have a build process with grunt that copies a bunch of files (and does some other work), and use grunt watch to keep those copies up to date. But sometimes it lags behind enough that it annoys me, like when I reload a test and I don't get the updated version of a file.

Using hard links instead of copies would alleviate that problem, as edits would be immediately/automatically propagated. It's kind of a hack, but it'd be helpful.

I suggested [adding this to contrib-copy](https://github.com/gruntjs/grunt-contrib-copy/pull/75), but it interacts with the options to grunt.file.copy, and would make more sense if implemented directly there. Specifically any time `processContent` applies to a file it shouldn't be hard linked.

The idea behind a hardLink option would be hard-link-if-you-can, and on platforms without that option it would do a copy, or copy when a hard link isn't possible (e.g., across devices).

(I can probably put together a pull request, but I wanted to open it up for discussion first to see if it'd be accepted.)