Load options from server instead of the config

[RasmusLindroth]

You can now retrieve data from the server and set some options more dynamically.

• [x] Chars per toot
• [x] Number of options in a poll
• [ ] Number of media attachments

https://github.com/trwnh/mastodocs/blob/7676a07a6444bfbd1246d29a6adce3686c234df7/content/en/methods/instance.md