Add ability to publish using 'npm version'

[JimiC]

Resolves #206
References #180

As per popular request vsce publish <version> will now use npm version <version>, in order to bump the package version. In the process, package-lock.json will also be updated (when using npm5+) and tagged git commit will be done (supporting gpg signing when it's set in git config).

Note: As before only major, minor and patch are supported as a valid <version>.

I refrained from implementing a rollback of the process, as if anything goes wrong after the version bump (i.e. network issues), the user can simply execute vsce publish (without <version>) and complete the publishing.

This implementation also does not interfere with using plain vsce publish via a CI service. The user, in that case, should perform npm version <version> locally. Then push the commit and the tag. The CI publishing stage should only call vsce publish -p <token>.

[joaomoreno]

Works great for me!