Add --require option
Add Offence#cop_name
Rework report classes into formatters
Add -f/--format option
Deprecate -e/--emacs option
Add -o/--out option
Refactor and style
Allow multiple formatters
Allow custom formatter class
Always pass absolute file path to formatter

Previously, path of files searched by RuboCop were absolute,
but paths which are directly specified in command line args were not.
Add YARD documentation to BaseFormatter