Support generic release hooks

[mpkorstanje]

🤔 What's the problem you've observed?

Not every project uses a well known language with a well defined release process that can be reasonably implemented in polyglot-release. Additionally some projects may want to do additional work in addition to the release process for their language. For example #56, were cucumber-jvm-scala needs to update some documentation references.

As such it might be useful to define a generic set of hooks that can be used in any project.

✨ Do you have a proposal for making it better?

We add a pseudo-language to polyglot release that consists of several optional bash scripts:

polyglot-release/
 |- before-pre-release
 |- pre-release
 |- after-pre-release
 |- before-release
 |- release
 |- after-release
 |- before-post-release
 |- post-release
 |- after-post-release

The before- and after- scripts are hooks that allow the opportunity to modify files before and after the language specific release process runs but before any commit is made. The pre-release, release and post-release are executed as part of the polyglot-release pseudo-language.

Each script is invoked with $1 = $NEW_VERSION and with pwd = [the project root]. So for example ./polyglot-release/release 1.2.3. And if possible the check_for_tools function is also exported.

📚 Any additional context?

In #65 we added a bespoke mechanism for polyglot release to release itself. This could be replaced by these scripts.