Feature: Support multiple installers

Fixes https://github.com/terlar/fry/issues/7

This don't implement `ruby-install` implementation yet and might need some revision to do that, but it uses a more flexible way to add build systems and have moved the `ruby-build` implementation to such a way.

This is how you add a new build-system:
Define method matching the name of the command you want to use as a installer (this is for the auto-dectection):
```sh
function fry-installer-ruby-build --description 'Installer for ruby-build'
  function __fry_install_ruby
    ruby-build $argv $fry_rubies/$argv
  end

  function __fry_install_rubies
    ruby-build --definitions
  end
end
```
Then the desired installer is chosen by:
`fry config installer ruby-build`