Add support for a {:struct, struct_name} type specifier

[josevalim]

Thank you @aselder!

Can you please:

1. Run mix format
2. Change the docs.ex and add tests to ensure we generate the proper docs for it

?

[aselder]

@josevalim Done!

[whatyouhide]

@aselder this PR is failing on Elixir 1.9 (still supported by this library) because is_struct/2 was introduced later on. Can you move the check to the body of the function, and use something like Map.has_key?(struct, :__struct__) and struct.__struct__ == mod?

[josevalim]

We can pattern match as well:

match?(%^mod{}, value)

[aselder]

Any idea what a rough timeline for getting this released is? Trying decide if I launch a feature with out and go back and update later, or wait for this to make it out. Thanks!

[aselder]

@whatyouhide Thanks Andrea