Support custom templates

[DanielKeep]

Allow users to specify their own templates for hosting --expr scripts. i.e.:

cargo script -tdisplay --expr 42

With something akin to this in $CARGO_HOME/script-templates/display.rs.template:

%p fn main() { println!("{:?}", %b); }

[DanielKeep]

Implemented in 0.2.5.