Option to use single quote

[neves]

https://github.com/bbatsov/ruby-style-guide#consistent-string-literals

Options:
  -q, --quote=<s>    Quote style ('single' or 'double') (default: double)

[joonty]

I wonder if a user-friendly default would be to check which quote style the Gemfile uses by majority. Obviously, double quotes are superior (🔥) but consistency is definitely the most important thing. Then we could add the command line argument as a way of forcing one or the other.