Parse RemoveFlag commands correctly.

Before this commit:
`> :flags remove 1`
will crash the repl with
`elm-repl: Prelude.read: No Parse`

Looking at the code history it looks like at one point "ResetFlag" took a number and now it takes a `String` but the parsing code didn't break because it uses `read` so there was no type error. The dangers of `read`!