Improve install integration with Literal gem

[gtarsia]

Does two things:

• Adds flag --use-literal-gem/--no-use-literal-gem (default true) to the generator phlex:install
• Adds the rails application template which asks if you want to use literal: if yes, installs literal, otherwise, uses the --no-use-literal-gem flag.

If you don't like where I placed the Rails application template (lib/templates/rails.rb) or want a more meaningful file name, let me know.

Comments:

• I made Literal opt-out because I think the experience is awesome for components.
• I will also push another PL for "Getting started" in the Phlex docs showing how to use this
• I tested this locally in a new project by including this in a new rails project Gemfile:

gem 'phlex-rails', path: File.expand_path("~/git/phlex-rails")

• It's kinda tricky to test otherwise because it runs a generator from the gem (which isn't up yet)

Anyone interested in using this should just add this to their Gemfile:
gem "phlex-rails", github: "gtarsia/phlex-rails"