ppx: Antiquotation in an attribute argument is currently unsupported

[Armael]

Using the ppx, the following pattern, which would be nice to have, is currently unsupported:

# fun extra_classes -> [%html {| <a class="foo bar |} extra_classes {|"></a>|}];;
Error: Error: Mixing literals and OCaml expressions is not supported in attribute values

[Drup]

I believe this one would be really useful to support. cc @aantron

[aantron]

Seems desirable. I likely won't have time to work on this in the near term.