Warn on unnecessary explicit return statement.

[cirosantilli]

https://github.com/polarmobile/coffeescript-style-guide recommends to only use return if necessary, i.e. not the last thing in the function.

The opposite of: https://github.com/clutchski/coffeelint/pull/30, if implemented here could be under a single return_style: [explicit|implicit|any] option.