Export depcheck for use in specials

[edj-boston]

It would be nice if gulp-depcheck exported the depcheck.specials property so I could do something like this:

gulp.task('package', g.depcheck({
    ignoreMatches : [
        'bootstrap',
        'font-awesome',
        'jquery',
        'npm-font-open-sans'
    ],
    specials : [
        g.depcheck.special['gulp-load-plugins'],
        g.depcheck.special.mocha,
        g.depcheck.special.babel
    ]
}));

As it is, I have to require depcheck itself to bring them in.

[goloroden]

Would you mind creating a PR for this?

[edj-boston]

Here you go: #9