Configurator

Adds a way to configure the annotation process with a `ActiveRecord::Annotate.configure` block with the first option adding triple backticks around the annotation as described in #2.

Checklist:

- [x] add `Configurator` object responsible for keeping all settings
- [x] extend `File#annotate_with` with conditional creation of backticks
- [x] provide a simple rails generator for a config file
- [x] describe all new functionality in the readme