Add support for more than one custom category

[Gargron]

Fix #224

[nolanlawson]

Thanks for the PR! Can you confirm that this isn't a breaking change, and could you add some docs to the README to explain how to use this new feature?

I ran the tests manually and they passed, although a new test wouldn't hurt either. :)

[nolanlawson]

Added some tests to confirm that this isn't a breaking change (if you don't specify customCategory, then the behavior is the same as before), and added a test and storybook for custom category as well as docs.

[nolanlawson]

Storybook with custom categories shown:

[lilong7676]

Thank you. Two questions:
How do I move a custom item to the front?
How to customize the width and height of the emoji icon?

@Gargron