feat: add support for skipping lifecycle steps, polish lifecycle work

[bcoe]

What is this?

this fairly major refactor addresses several feature requests around lifecycle hooks, and the ability to skip steps that standard-version performs.

There are now the events bump, changelog, commit, and tag, each of which has preandpostlifecycle hooks, e.g.,prebump, postbump, precommit`, ...

fixes #157

Note

I've branched from #187 which should be landed first (so this commit looks bigger than it actually is).

CC: @lancecaraccioli, @denouche,

[coveralls]

Coverage decreased (-0.1%) to 99.503% when pulling 1e45bed on skip-steps into dfd1d12 on master.

[coveralls]

Coverage decreased (-0.1%) to 99.503% when pulling 1e45bed on skip-steps into dfd1d12 on master.

[coveralls]

Coverage decreased (-0.1%) to 99.503% when pulling 10f3e03 on skip-steps into d073353 on master.

[bcoe]

I've pushed this to next, and have been testing on marky-markdown which has complicated build step:

https://github.com/npm/marky-markdown/blob/master/package.json#L14

I'm really happy!

✔ bumping version in package.json from 10.1.1 to 10.1.2
✔ Running lifecycle script "postbump"
ℹ - execute command: "npm run build"
✔ outputting changes to CHANGELOG.md
✔ Running lifecycle script "precommit"
ℹ - execute command: "git add marky.json"
✔ committing package.json and CHANGELOG.md
✔ tagging release 10.1.2
ℹ Run `git push --follow-tags origin master; npm publish` to publish

[coveralls]

Coverage decreased (-0.1%) to 99.502% when pulling 37a2647 on skip-steps into d073353 on master.

[denouche]

Nice!! Thank you!

[denouche]

Maybe you could precise in the readme that the output of the precommit hooks is used as commit message. Because it's not obvious. By the way I'm not sure that this behavior is relevant, but it's you to decide. At least to have it in the documentation to avoid errors on releasing without understanding why.

Thank you!