Updated signature to handle map special forms

*Note:* Made a new `v0.13` branch to capture all Erlang v0.13 changes.

Updated `ExDoc.HTMLFormatter.Templates.signature()` to handle the two new map special forms: `%{}` and `%Struct{}`. This allows `make docs` to run successfully in the Elixir `v0.13` branch.

Might need to create new specific cases to make the signatures better. Willing to take input.