"keybase execute": replacement command for 'curl | sh'

I would really like to be able to tell someone to run the command

```
keybase execute glyph https://glyph.im/some-code.sh
```

to replace telling them to pipe the output of `curl` to the output of `sh`.

The present UI for doing this is insufficient, because `keybase verify` doesn't un-base-64 its input and `keybase decrypt` happily outputs unverified data before it ultimately fails, which doesn't help in the `|sh` pipeline.

Start by reviewing the existing keybase verify and keybase decrypt command behavior described in the issue, then identify the command entry point for a new keybase execute operation. Done means a remote script can be decoded, verified, and executed without piping curl output directly to sh, while unverified data is not sent to the shell.

Written by the indexing model from the issue text.