Add support for specifying features with commas feat1,feat2

[dmatos2012]

Fixes #915 by enabling support for specifying features via commas. Specifically it adds support for the flags:

• --baseline-features
• --current-features
• --features

Before:
cargo semver-checks --baseline-features foo,bar
Raised:

warning: Feature `foo,bar` is not present in the baseline.

Now:
cargo semver-checks --baseline-features foo,bar
Succeeds fine.

Thank you! :)

[obi1kenobi]

Thank you!