add subcommand

Adds the specified gem to Gemfile (if valid) and run ‘bundle install’ in one step.

$ bundle add GEM VERSION

Example:

$ bundle add jekyll-optional-front-matter

Line appended to Gemfile:

gem "jekyll-optional-front-matter", "~> 0.3.2"

Output:

Fetching gem metadata from https://rubygems.org/..........
...
Using jekyll-optional-front-matter 0.3.2