Custom buttons for textareas#76

It would be nice if custom buttons for textareas could be defined by plugins, e. g. following the scheme defined by the Editor field.

See https://github.com/getkirby/ideas/issues/210.

That would be great when we have our own custom kirbytags, that we want the client to use - clicking a textarea button would be much easier than having to remember the syntax.

An agency I’m working with really loves NO-BREAK SPACE & SOFT-HYPHENS to better control typography. In Kirby v2 it was beautifully simple to add buttons for these to the textarea toolbar.