[Feature] Support parsing commit message body

[yanang007]

Hi! I really like the idea of gitmoji-changelog, but I'm recently encountering a strange situation. For example, a commit that adds a feature using APIs from a higher version of upstream and therefore drops support for older version. IMO the commit message may be something like:

:sparkles: Add new features

- :boom: Remove support for older versions

However, for now gitmoji-changelog parses only the subject of commit message, the 💥 down there will be ignored. So I'm wondering if some gitmojis can be declared to be CRITICAL that should be extracted from commit message body and then be treated just like normal subjects.

I have implemented a demo in my fork (yanang007@06ebe74) , which adds a dummy group "critical" like "useless" to mark critical gitmojis and the "critical" group will be removed before mapping gitmojis to ensure consistent behaviors. Implementing the idea like this can reuse the custom group mapping mechanism and provide more flexibility.