Support description option body_param

This is a proposal to fix #42. See the included commit message for details.

With this change, we can at least have textarea input for complex type that couldn't be represented with form fields.

The included parameter names and descriptions are lost. We could try to put them in some formatted way into `notes`, but since there's no guarantee that it supports markdown, the result would be messy.

Ideally, this should be eventually solved in #39.