Add Microscope.configure to configure special verbs

*Extract from README.md*:

> Microscope uses a rather simple process to convert field names to infinitive verbs. It just removes the d if there’s one at the end of the verb. It also has its own irregular verbs list.
> 
> For example, a `liked_at` field name will give `like!` and `unlike!` instance methods to records.
> 
> But you can configure Microscope to use your own special verbs. For example:
> 
> ```
> # config/initializers/microscope.rb
> Microscope.configure do |config|
>   config.special_verbs = { 'extracted' => 'extract', 'started' => 'start' }
> end
> ```