ability to exclude certain key names from `kapp config-map`

**Describe the problem/challenge you have**
Knative's config maps have an `_example` key and they're multi-line and noisy and there's not way to exclude them from the `kapp config-map` output

**Describe the solution you'd like**
I was thinking a `--exclude-keys key1,key2` flag would work

**Anything else you would like to add:**
Thanks for the help!