Add a Ronin::Recon::Config#save method

[postmodern]

Add a method which writes to_yaml to a file path. This will act as a convenience method for saving modified config objects back out to the default ~/.config/ronin-recon/config.yml config file.

def save(path=DEFAULT_PATH)
  File.write(path,to_yaml)
end

[postmodern]

Implemented by PR #162.