Implement --diff option

Closes #7
Remove --dry-run cmd-line option

--diff should suffice, better remain lightweight for now
Perform rename in also in file names

Fixes dkrikun/rename#10
Remove --ack cmd-line arg

Closes dkrikun/rename#11
Implement is_snake_case()
Implement is_camel_case()
Implement camel <-> snake conversions
Implement across-case edit_line()

With snake, camel and all caps case.
Closes #9
Implement --word and --almost-word

Closes #5
Closes #6
Satisfy pylint