extract func `parseFormTag` from templatefunc.RenderForm

Extracted a func `parseFormTag` that takes a reflect.StructField
and returns the different positional parts of the `form` structTag
with default values as documented in
http://beego.me/docs/mvc/view/view.md#renderform

This makes RenderForm shorter and makes it possible to test the
parsing separately.
Removes unused FormType map from templatefunc
templateform.RenderForm now renders textarea

When RenderForm encounters a field with the structTag `form` value
type `textarea` it renders an actual <textarea> html tag.
Merge remote-tracking branch 'origin/renderform-textarea' into develop