Allow cargo-script to run programs directly from online package source.

[DanielKeep]

Presumably, this would be crates.io and possibly GitHub and/or raw Git urls. Something like:

• cargo script pkg:some-tool - fetch, build and execute the latest version of the some-tool package from crates.io.
• cargo script pkg:many-tools,frob - fetch and build the latest version of the many-tools package from crates.io, and run the frob binary.
• cargo script gh:DanielKeep/cargo-script -- script pkg:many-tools(1.1),glom - fetch, build and execute the master branch of the DanielKeep/cargo-script repository from GitHub, which will then fetch and build the many-tools package from crates.io with version spec ^1.1, and then run the glom binary.

[DanielKeep]

See #7.

[DanielKeep]

Given that cargo install means we're starting to get more actual binaries on crates.io, this should be even more useful.