Major Chestnut relase upcoming

Update: I’ve merged the re-frame PR for lack of feedback. I suppose no-one objects.

I made a handful of other changes that jumped out on me, like cleaning up the less/sass stuff (ugh boy is that an ugly hack).

The system constructor now takes a config map. Adding a Code of Conduct is now optional.

browser-repl will become cljs-repl to be more in line with what others are doing. The browser-repl alias will stay around until Chestnut 1.0. The same for run which is now go.

Comments more than welcome, either here or on the pull request: https://github.com/plexus/chestnut/pull/197