Add version bumping process

[Gary Jones]

There are a few places where version numbers should be increased for new releases, and some of them have been missed (such as for 1.9.5 (See #618) and 1.9.7).

Add a version-bump-prompt npm devDependency, and a small set of scripts to target the correct files (see https://github.com/Automattic/Co-Authors-Plus/blob/develop/package.json as an example) to handle this.