Add endpoint /forms/post with simple HTML form that submits to /post

I'm writing a mechanize clone and would like to test how form submission. See also  https://github.com/kennethreitz/httpbin/issues/118

Possible objections:

1. This is beyond the scope of httpbin. Perhaps it would be better on a new website htmlbin.
2. The example form given isn't thematically in keeping with the rest of the site because it's about pizza.