Add support for plugins

I've also did not find anything which scares me. Block indentation and line continuations are not the same and should not be indented the same. Otherwise you get stuff like:
```ruby
if long_multiline_condition &&
  condition continues
  body_of_the_if
end
```
instead of 
```ruby
if long_multiline_condition &&
    condition continues
  body_of_the_if
end
```
(maybe not nice but at least there is a difference)

Having same indentation is a terrible idea in the Ruby community and I'll fight for 2/4 spaces!
Will you budge? :) Or will I have to update? :(

Anyway I'll open PR to https://github.com/bbatsov/ruby-style-guide/blob/master/README.md this should change, but I am afraid the battle may be lost.