feat: add prerelease lifecycle script hook (closes #217)

[dmfay]

It took me a while to get around to it, but here's an implementation of a pre-everything script hook that won't affect behavior further into the process.

[coveralls]

Coverage remained the same at 100.0% when pulling 272e2fd on dmfay:prerelease into 9a99607 on conventional-changelog:master.

[dmfay]

hey @bcoe -- any chance of this making it in sometime soon? :)

[bcoe]

@dmfay I apologize for the slow turn around ... 2018 has been a bad year for OSS for me.

Refresh me, wouldn't attaching an event to prebump be equivalent to prerelease? What are the advantages of this; I suppose the fact that returning a version # doesn't have an effect on releaseAs?

[dmfay]

No worries, these things happen. There's more information in the original issue but you've got the gist of it: any and all output from prebump is parsed for a version number, which makes it difficult to execute anything that produces its own output.