Rework report classes into formatters
Add -f/--format option to specify formatter
Deprecate -e/--emacs option
Add -o/--out option to specify output file
Refactor and style
Enable multiple formatters
Enable use of custom formatter class
Always pass absolute file paths to formatter

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