Add subcategory support to the generate command

[remilapeyre]

This patch adds a --subcategory flag that let users give a mapping that will use to set the subcategory field in the documentation:

tfplugindocs generate --subcategory consul_acl=ACL --subcategory consul_admin="Admin Partition"

The format for the flag is prefix="Sub Category" so all resources and datasources starting with consul_acl like consul_acl_policy, consul_acl_token, etc. will have the subcategory: "ACL" in the generated documentation. This is not very elegant but should work in most cases as needed.

Closes https://github.com/hashicorp/terraform-plugin-docs/issues/156