Add option to support publishing to npm

[liddellj]

Presently I'm using a combination of this task and grunt-release because I need the publish to npm functionality from grunt-release, and I need the updateConfigs option from grunt-bump (which grunt-release does not support).

If grunt-bump provided an option to publish to npm, I could drop the dependency on grunt-release.

[eddiemonge]

So the only thing that grunt-release adds on this is

• create a .zip release on github
• publish the new version to npm

Those two tasks could be separate tasks that are run after this. If those were added then this becomes grunt-release, which already exists.

[eddiemonge]

See geddski/grunt-release#85