Add --group-features option

[taiki-e]

Closes #80

--group-features <FEATURES>...
    Space-separated list of features to group.

    To specify multiple groups, use this option multiple times: `--group-features a,b --group-features c,d`

    This flag can only be used together with --feature-powerset flag.

$ cargo hack check --feature-powerset --group-features a,b
info: running `cargo check --no-default-features` on crate1 (1/5)
...
info: running `cargo check --no-default-features --features c` on crate1 (2/5)
...
info: running `cargo check --no-default-features --features a,b` on crate1 (3/5)
...
info: running `cargo check --no-default-features --features c,a,b` on crate1 (4/5)
...
info: running `cargo check --no-default-features --all-features` on crate1 (5/5)
...

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci

[jhpratt]

Amazing speed! Thank you!

[taiki-e]

Published in 0.4.1.