Implement functional variant of the styled macro

[tecosaur]

I think it would be rather nice to be able to take a "styled representation" (e.g. {magenta:hey}) and transform it to an AnnotatedString as the styled"" macro does, but with a function.

This would require a major refactor to stylemacro.jl, and I think we'd want to chop out the code-evaluation related functionality from the functional form, but I think it would be worth having.

[tecosaur]

I'm now trying to create a PR-ready branch in https://github.com/JuliaLang/StyledStrings.jl/tree/functional-styled-markup, I'll make a PR as soon as it seems viable and of sufficient quality.

[tecosaur]

Implemented in #43