JSON parsing and version option

As I've mentioned in https://github.com/stevelacy/gulp-bump/issues/1, I propose to use JSON.parse/JSON.stringify to modify only the top-level version.
This plugin is only ever usable for JSON as I understand and it sounds sane to limit bumping to the top-level version option inside a JSON file. What needs to be done with possible other version options is hard to decide from within this plugin.

Secondly, I've added a `version` option to specify a specific version which you want to "bump" your JSON files to.

What do you think?