Add `spring.shell.enabled` property for global en/disable

[onobc]

Maybe we should add spring.shell.enabled property as traditionally most of a boot autoconfigs have global flag for feature.

Originally posted by @jvalkeal in #374 (comment)

[ullenboom]

Is this still on the table?

[onobc]

Hi @ullenboom , we still want to do it but we have not had time to do it. We would happily accept a contribution. :)