[Feature Request] Support GNU Hyperbole explicit and labeled implicit buttons

[rswgnu]

Hopefully you know that GNU Hyperbole is an extensible hypertext framework with easily identifiable, textually delimited explicit buttons. In the latest pre-release (available on Github and elsewhere), there are also identifiable labeled implicit buttons that use <[label]> as delimiters. All these buttons can appear in any type of file or buffer.

I hope you will add support for these. You could limit activation to only when hyperbole is loaded in which case (featurep 'hyperbole) is t.