[feature] Conversation : Add/remove nested resource 'contact'

[WaKeMaTTa]

Implemented add_contact and remove_contact actions in Conversation.

# Add a contact to a conversation
conversation.add_contact(admin_id: admin.id, customer: { intercom_user_id: contact.id })

# Remove a contact from a conversation
conversation.remove_tag(id: contact.id, admin_id: admin.id)

Fixes #515

[murtron]

Nice one @WaKeMaTTa, this is great! Just one minor README change and I'll include this in the next release.