Allow styling text

Check out https://void.lgbt/objects/da64778e-033e-43df-9ed8-d91b9ed08ea8

It has bold & italic text, which Phanpy renders nicely. Apparently this is done with a limited subset of HTML tags. A few Fedi packages, e.g. the Glitch fork and Akkoma, allow authors to style posts (usually with markdown I think). I think if Phanpy started offering the opportunity to style text, this would really be attractive to a lot of people.