Allow constructor to be called as a function

[jordaaash]

Idiomatic use for gulp seems to be to pipe to the output of a function call, which also works more naturally with requires that need a setup.

Current behavior (throwaway variable require):

var RevAll = require('gulp-rev-all');
var revision = new RevAll({ /*options*/ };

Current behavior as one-liner (non-idiomatically invoked require):

var revision = new (require('gulp-rev-all'))({ /*options*/ });

New proposed behavior (idiomatic immediately invoked require):

var revision = require('gulp-rev-all')({ /*options*/ });

What do you think?

[jordaaash]

Thanks @mix3d!

Edit: And @smysnk 💯