Exit with nonzero status on warnings

[dmurvihill]

Thanks for your work on this package!

I'd like to have a CLI flag and config option to exit with a non-zero status if any warnings are emitted for invalid commit messages, for use in CI.

[qoomon]

I'm lacking a little bit of time however, every PR is welcome.

[qoomon]

In this parsing method we need to gather and throw all warnings as one.

git-conventional-commits/lib/gitCommitConvention.js

Line 81 in 682d15e

function parseCommitSubject(commit) {

and then we need to handle exceptions everywhere we using this functions. Depending on a cli parameter we would the print the warnings or throw an error.