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.

[hsbt]

@byroot With this change, a task whose name ends with ? and an argument ending with ? (e.g. rake "search[what?]") silently stop working, since the trailing ? is chomped before parsing.

I have rarely seen such names in real projects, so I could accept this as an incompatibility, but is that breakage intended, or do you plan to handle these cases?

[byroot]

but is that breakage intended,

I didn't think those were valid names. If it is a concern, I can look at another symbol that isn't currently a valid name if there is one.

But as you suggested, I can also look at handling the ambiguity of it. Just let me know what you prefer.

[hsbt]

Sorry for the slow reply. Let's keep ? and handle the ambiguity. Defining a list of valid task name characters now would mean adding a fallback every time someone turns out to be using one, so I'd rather cover just these two cases and leave the rest alone.

[byroot]

@hsbt no worries. Reopened at #765 with the requested changed (because this PR was done with a deprecated account).