Add safe invocation CLI syntax

[casperisfine]

Example: rake may:or:maynot:exist?

Such syntax is useful for projects like buildpacks and other automated CI/CD tools.

It would allow these tools to very easily invoke a taskif it exists, without failing if it doesn't.

@hsbt is this a feature that could be acceptable?

Also cc @schneems, since as maintainer of https://github.com/heroku/heroku-buildpack-ruby I suspect such feature could be useful.