[FEATURE] decrement/truncate

[isaacs]

Not pressing or urgent, but mostly for API symmetry, it'd be nice to have truncate/decrement functions.

1. semver.truncate('1.2.3-foo', 'patch') == '1.2.3'
2. semver.truncate('1.2.3', 'minor') == '1.2.0'
3. semver.truncate('1.2.3', 'major') == '1.0.0'
4. semver.dec('1.2.3-foo', 'patch') == '1.2.2'
5. semver.dec('1.2.3', 'minor') == '1.1.0'
6. semver.dec('1.2.3', 'major') == '0.0.0'

Re #46, but the use case presented there turned out to not require this functionality. It's still probably useful, though.

cc @ronkorving

[aschearer]

Would be handy to have semver.dec. I would like to generate a change log from git based on the current version and the previous version. Not sure of a way to get the previous version short of decrementing the package's version.

[sindresorhus]

I didn't have time to do a proper patch, but I did a quick module for truncation if anyone else needs it: https://github.com/sindresorhus/semver-truncate

[anko]

What should semver.dec('2.0.0', 'patch') be? 1.99.99? 1.0.0? Something in between? The semver spec doesn't define how to decrement a semver or what it even means. I suspect this is why.

[ypicard]

Any news on this?