Scrolls::Decorators

This is a proposal for an alternative (and simpler) frontend for `Scrolls::Log`. It should cover the common cases, but the underlying `Scrolls::Log.log` is still there when needed.

See [test_decorators.rb](https://github.com/fabiokung/scrolls/blob/master/test/test_decorators.rb) for usage.

Example:

```ruby
class MyService
  include Scrolls::Decorators

  log_context "myservice", release: "v51"

  def simple_log_with_context
    log
    # output: "myservice release=v51\n"
  end

  def basic_logging
    log "basic", "that", "works", data: "useful", number: 10
    # output: "myservice release=v51 basic that works data=useful number=10\n"
  end

  log
  def wrapping
    sleep 0.1
    # output:
    #  "myservice release=v51 class=MyService method=wrapping at=start\n"
    #  "myservice release=v51 class=MyService method=wrapping at=finish elapsed=0.1"
  end

  # and more ...
end
```