Allow grammars to specify tab style

GNU Make requires the use of hard tabs, but our current setup doesn't allow grammars to specify tab styles. This PR will allow grammars to specify 'requiresHardTabs' which will override any other softTab preference.

**BUT**

I'm wondering if this approach is too shortsighted. I like what @jtkiley recommends in https://github.com/atom/settings-view/issues/122#issuecomment-44845687. But that will clash with the grammar approach I took here.


Started because of https://github.com/atom/language-make/issues/3 and https://github.com/atom/language-make/pull/8/files