Introduce `suspenders:inline_svg` generator

[crackofdusk]

Ports the existing generator to Suspenders 3.0.

To do:

• [x] Add documentation
• [x] Manually test the generator against a new Rails app

How to test manually

Add suspenders to the Gemfile.

group :development, :test do
  gem "suspenders", git: "https://github.com/thoughtbot/suspenders", branch: "suspenders-3-0-0-inline-svg-generator"
end

Install and run the generator:

bundle
bin/rails g suspenders:inline_svg