Add an option to not add a trailing comma to the wildcard arm

[sanxiyn]

By the wildcard arm, I mean _ => .... Trailing commas are wanted because one wants to add new arms below without touching old arms. But the wildcard arm is necessarily the last arm and no new arms will be added below.

This is similar to #712 but different.

I observed this to be an existing practice while going through Manishearth/rust-clippy#540.