Hide some flags/features from gofmt

[mvdan]

We are backwards compatible with gofmt in terms of formatting, and in general we want to be a drop-in replacement.

However, some gofmt flags and features don't really belong in gofumpt. For example:

• The rewrite flag, -r, isn't used by many people. Those who depend on it should just use gofmt -r.
• The simplify flag, -s, is on by default in gofumpt. It makes little sense to allow disabling it. The only use case I can think of is the Go compiler itself, since it needs to bootstrap from a very old version of Go, which might break if -s removes some types which later Go versions made optional. But we could always fix that with the -lang flag.
• The -cpuprofile flag is an internal performance tool, so it shouldn't really be exposed. The -e flag probably fits the same bill, but perhaps some users actually make use of it.