Add support for multiple Types to get grouped together, so my devs can use their own preferred type selector and still generate the same changelog.

Expected Behavior

Ability to let my devs use both "feat:" and "feature:" while grouping them together in the changelog by Title from the title_maps.

Actual Behavior

Every type shows up in a different heading, even though the types that ought to be grouped are mapped to a string exactly the same as others.

Steps to Reproduce (including precondition)

Add Types that you want to group in the same heading, and look for a way to do that. Find nothing because the logic doesn't allow for it.
{Please write here}

Your Environment

• OS: Ubuntu 18.04
• git-chglog version: 0.7.1

I understand the solution from the author's perspective would be to force the devs to standardize their Type tags, but I would prefer to offer my team the flexibility to not remember "bug:" and "bugfix:" don't do the same thing in the changelog.

If I have time on this project (haha that's a good one) I'll attempt to add error checking into the md template to group exact matches together. It seems like it wouldn't be hard to change the type on the commit_groups.title_maps to accept an array/slice of strings instead of a single string to the match. Might make sense to do a bigger refactor and set commits.filters.type to be an array of string([]string) maps with the desired heading titles as the keys and an array of possible type matches as the values.

Let me know if this is a worthwhile update - if my devs rebel too much, I may fork and implement one of these options, but that would be a refactoring project for sure.