💡 simple support for themes and customization

[breck7]

if a folder has a hakon override, use it (perhaps can be fetched in readme.scroll and/or individual scroll files, not sure). same for stump (but think this through).

same thing for grammar.

[breck7]

You can now extend grammar by dropping in a grammar file. And you can do basic customization to header and footer via scroll.settings.