elm-review new-rule should not clobber existing files

[SiriusStarr]

Currently, it seems that elm-review new-rule RULE will clobber any extant files at src/RULE.elm or tests/RULETest.elm without any warning. This is dangerous if one is working on a new rule and arrows up one too many times in one's shell history. Don't ask me how I know this. 😆

Regardless, it should probably warn if the files already exist rather than just overwriting them.