Support arbitrary cargo commands.

[Sean1708]

It'd be great to be able to write something like

script:
    - travis-cargo --only nightly clippy --lib

to run clippy on nightly. You could probably just leave arguments unparsed and pass them straight through.

[huonw]

I suspect this would be fixed by #19. The current scheme of hard-coding cargo subcommands is a hack because I couldn't get anything else to work at all well (and I didn't know about the functionality mentioned in #19 at the time).

[huonw]

I've manually added support for travis-cargo clippy in 9b73cda (and travis-cargo fmt), but they're special cases.