Allow customizing environment text header

[tobydriscoll]

Is your feature request related to a problem? Please describe.

A prf environment always puts its title inside parentheses.

Describe the solution you'd like

I'd like the ability to customize the prefix and suffix of the title, e.g., "Theorem 1: Abel's formula".

Describe alternatives you've considered

I tried using CSS before: and after:, but the parens are hard-coded into the HTML. With the old sphinxcontrib-proof package, I was able to change the templates in config.yml, but this was undocumented and kludgy.

Additional context

Thanks! This package was badly needed.

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