Added support for multiline commit messages

[niclashoyer]

I ran into an issue with multiline git commit messages. I wanted to add [ci skip] to a release commit. I tried setting

release.options.commitMessage = 'Release <%= version %>\n\n[ci skip]';

which indeed printed out the following in verbose mode

Running: git commit package.json -m "Release 0.0.2-5

[ci skip]"

but it seems that shelljs.exec has problems with multiline commands, as the execution just hangs and never completes, which also causes timeouts on travis builds. It may be a bug in shelljs, but an answer on StackOverflow also stated that it is much better to use the -m flag instead of newlines.

So I added the ability to pass an array to commitMessage where each element will be parsed as an argument to git commit with the -m flag which sets multiline commit messages accordingly (even on windows).

[drublic]

Thanks for the PR. It landed in 5c10148.