Get max_toot_chars from instance

[omnivagant]

Some implementations, like pleroma and glitch-soc, implement and show the client max_toot_chars. I'd suggest to try to use that if char-limit is not set, assuming there are other implementations and having char-limit configurable in tut would still be desirable.

Example of what the python client toot does: https://github.com/ihabunek/toot/blob/master/toot/tui/app.py#L320

[RasmusLindroth]

It's now fixed in the latest release :)