Plain js source code tiddler support

Hi @Jermolene, this change set adds support for saving *plain* JavaScript source code files with embedded meta-data comments sections. No more `.js.tid`, `.meta` sidekicks, `tiddlywiki.files`, ... for those that want to create their TW5 modules from within TW5.

I've also thrown in tests for the new `<$metafy>` widget and the associated `plain-javascript-tiddler` template. And last but not least you even get tw5.com documentation for the `<$metafy>` widget on top. Enjoy.
Good catch. I'll manually commit this fix separately so that it doesn't have to wait for the rest of the pull request.