Rely on local specifications only

This implements the idea outlined in #14: rely on Bundler (and in turn RubyGems) and its locally installed specifications to read summaries from.

Advantages:

- Supports all source options, such as `rubygems`, `git` and `github` (fixes #10)
- Lightning fast as it reads from disk
- It fetches summaries for gems that match version requirements
- Piggybacks on Bundler to load/parse gem specifications

Downside:

- Will only work for projects using bundler
- Running `bundle install` prior to running `label` is a requirement

There's one thing withstanding here and that is to make sure we use the exact version that ends up in the `Gemfile.lock` file. I'm not too sure if there's a Bundler runtime method available for us to do so.