feat: new option transformCommits to postprocess commits

[ext]

Adds a new option transformCommits to postprocess commit list before release notes are generated.

While it is possible to create a custom reader it might not always be desirable to maintain a forked to only augment slightly. Additionally the reader is only passed the commit message and not the full parsed data such as SHA. My personal use-case is to automatically set the scope of the commit in a monorepo setup based on which packages the commit touches.

The option can either be a function (usable when using a .js configuration file) or anything resolvable using require which default exports a single function. In both cases the function takes a single commit object (same object as is passed to conventional-changelog-writer later) and expects it to return a similar transformed object.