Features from optional dependencies

crossterm implicit feature

Enables crossterm ^0.25

crossterm:

A crossplatform terminal library for manipulating terminals

Affects requestty-ui::features.SNAPSHOT_PATH, backend::get_backend, events::get_events

termion implicit feature

Enables termion ^1.5

termion:

A bindless library for manipulating terminals

Affects backend::get_backend, events::get_events