Allow to use custom reporters

[fatkodima]

Fixes #1.

It should be used like

class MyReporter
  def report(title, created_records)
    # do actual reporting
  end
end

ColumnsTrace.reporter = MyReporter.new

cc @andriy-baran Wdyt on the implementation?

[andriy-baran]

Looks great. Only thing to improve is documentation
Something like this:

class MyReporter
  def report(title, created_records)
    title # => 'controller#action'
    created_records # => [<#ColumnsTrace::CreatedRecord>]
    created_records.each do |record|
      record.model # class of AR model
      record.accessed_fields  # array of fields
      record.unused_fields # array
      record.backtrace # string
      record.record # AR model instance
    end
  end
end

[andriy-baran]

Do we need to call accessed_fields before unused_fields to use it right, or order does not matter?

[fatkodima]

It should not matter, I pushed a small fix for this to actually be the case.

[fatkodima]

Thanks for review! Improved docs.