Allow using script with a template instead of just expr

[jeromegn]

Maybe this is already a feature, but I can't figure out the syntax.

I made a template called sys.rs and put it in the right path, but none of these work:

rust-script myscript -t sys
rust-script -t sys myself

--expr seems required with a --template flag.

[fornwall]

Perhaps this is good, to avoid making scripts non-portable?

Perhaps specifying a dependency to a local crate could provide some similar functionality for those who need it, to avoid having something rust-script specific?