Add support for arbitrary cargo subcommands

[frengor]

This PR adds the all-features subcommand to allow cargo-all-features to work with arbitrary cargo subcommands like Clippy and Miri.

Syntax:

cargo all-features <CARGO SUBCOMMAND> <CARGO FLAGS>

This also adds the --feature-flags-last flag to better support tools which requires a subcommand, like Miri. The flag makes cargo-all-features to put feature-related flags (--no-default-features and --features) after the additional cargo flags passed by the user. Thus, for example, to call Miri one could run cargo all-features --feature-flags-last miri test <OTHER MIRI FLAGS>.
The old binaries haven't been removed.

Fixes #5
Fixes #15
Fixes #26

[frengor]

@frewsxcv @Manishearth @robertbastian (sorry for the ping) Can this be reviewed?

[Manishearth]

I think this is probably a good idea in the long run but I don't have the time to review this right now, sorry.