Make opts argument optional

[hschne]

This fixes #23 and reverts #31.

As discussed in #31, visualize_api() now accepts an optional argument opts. If no opts are passed, we fall back to an empty table. The Readme and annotations have been updated accordingly.

Additionally, I added a stylua config and formatted the code. In particular requests.lua had some code alignment issues that are now fixed. Formatting is a separate commit, for opts & readme change see 615bef1d178c5c46f21cea5255cee8626d15fdfb.

P.S. I really like this plugin and use it pretty much daily. Thought since I'm opening a bunch of new issues I might as well fix some old ones. Hope it helps 🤗