Add new config "commit-message"

[nochso]

Hi! I've added a new config option, passing tests included.

To test the commit message used, I modified fake.sh to redirect the message to a local temp file.

It should be fully backward compatible, as it falls back to the current default message.

Taken straight from commit.md:

---

commit-message

This option can override the first line of the default commit message. If the setting is omitted or the value is empty, the default message is used:

Update dependencies

[nochso]

Oops, this is not valid in PHP <5.6

    const TEMP_DIR = __DIR__ . '/temp';

I'll move it to a property instead.

[pyrech]

Hi :)

Thanks for your PR. It looks good to me. Before I merge it, could you stash your commits?

[nochso]

I'm assuming by stash you meant squash? Let me know and I'll squash it into a single commit and move the single messages into the commit description.

[pyrech]

My bad, yes I meant squash, sorry :wink:

[nochso]

Ok, I squashed all commits and force pushed it.

[pyrech]

Awesome! Thanks again @nochso