Adds a new configuration option that allows you to bump the version number in the package.json being updated. e.g. you might configure Renovate to bump a patch every time, or maybe for dependencies only and not devDependencies. e.g. if you configure `"bumpVersion": "patch"` then a Renovate PR updating a `package.json` that used to be version `1.2.1` will now see it updated to `1.2.2` (in addition to the dependency version(s) being updated too). Thanks to @gunar for the feature suggestion.

Closes #861