Would a PR making custom compat-tables easier be welcomed?

Hi,

I'm wondering if a PR that would make building custom compat-tables easier be welcomed.

I'd like to add the following options to build.js:
- specify which file to use instead of environments.json
- ability to exclude the testresults for the current browser

And then changes to build.js/master.js to hide non-applicable stuff from the UI, like:
- engines from the legenda that aren't referenced in the provided environments file
- unused platformtype headings
- unused obsolete/unstable checkboxes
- etc

Currently at least NodeJS and Rhino hosts their own version of compat-tables (containing just NodeJS/Rhino versions), based on a very old version of this repo.

Not sure if NodeJS would switch, but for Rhino I'd rather be able to build our custom compat-tables using the latest version of this repo, instead of some modified fork.

I did some prototyping and got the basics going, but before putting in the effort to make it a polished PR, I'd like to know if it has any chance of getting accepted.

Paul

Start by reading build.js and master.js, then trace how environments.json feeds the build and UI. Compare the proposed custom environment-file and current-browser exclusions with the existing compat-table flow; done would mean custom tables build successfully and omit unreferenced engines, platform headings, and obsolete or unstable controls.

Written by the indexing model from the issue text.