add the `--sign` option to the `bundle` command

[beroal]

Since the bundle command writes the bundle to the file system, why not allow a user to sign the bundle in the same command?

[conrad-heimbold]

I guess it's because of the good-old UNIX philosophy:

1. Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new "features". That's why bundle only does the bundling, but not the signing.

2. Expect the output of every program to become the input to another, as yet unknown, program. The output of the bundle command can be used as the input of the sign command.

Although it might be useful to incorporate that feature into the "bundle" command, I don't think it is a good idea to sacrifice the UNIX philosophy.

[beroal]

@conrad-heimbold all these commands are executed by one program sbctl.