Add check for unless to GuardClause and make one-line body configurable

Until now the cop only checked for `if` - now it also looks for `unless`.

Also now its configurable if the cop should ignore one-line bodies or not. I think by the default it should not, because this is still shorter / better to read:

```
# bad
def foo
  if bar
    baz
  end
end

#good
def foo
  return unless bar
  baz
end

# also good
def foo
  baz if bar
end
```