merge options.alias with options.require

Following this issue #230, I thought it could be a good idea to merge `options.require` and `options.alias`. It seems unclear to have 2 different options to do the same thing in browserify.

In browserify cli, with or without an exposed name, the option is still `-r myModule` or `-r myModule:myCustomName` so having the same feature in grunt-browserify would be nice.

Also, I didn't change the documentation to reflect this change. I think we should drop the alias option in the doc and only mention remapify to do dynamic alias mapping. To me when I first use grunt-browserify, it was unclear if I could have an alias without the plugin.