Adds support for niche.com transaction types

Fixes #16 

Adds support for niche.com transaction types:
- user_welcome-niche: General welcome email to niche.com users
- case user_password-niche: email informing the user of their temp DoSomething user password.
Maybe consider making these tags just use underscores (`user_welcome_niche`, `user_password_niche`) to match the conventions used with the other tags in this file.