feat(build): Add a `grunt version` task to stamp a new version.

* Adds grunt-version and grunt-conventional-changelog dependencies.
* Updates the version number in package.json
* Updates CHANGELOG.md
* Creates a tag.
* Commits everything.

NOTE: for the CHANGELOG.md extraction to work correctly, [these conventions](https://github.com/ajoslin/conventional-changelog/blob/master/CONVENTIONS.md) and [expanded conventions](https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit#heading=h.8gbcep5xnw19) for commit messages should be followed.

Closes #496