Breaking changes

• Split library into a backend-agnostic cursive-core and a user-facing cursive.
• Cursive::default now needs the cursive::CursiveExt trait to be in scope.
• Update dependencies:
  • crossterm to 0.17.
  • enumset to 1.0
  • ahash to 0.3
  • pulldown-cmark to 0.7
• Add PaletteColor::HighlightText
• AnyCb now takes a &mut dyn View rather than a &mut dyn Any.

API updates

• Added cursive::{default,ncurses,pancurses,termion,crossterm,blt,dummy} functions.
• Add Cursive::debug_name
• Add ScreensView to move some code away from the Cursive root
  • Reworked global callbacks configuration
  • Ctrl-C can be rewired to no longer exit the application
• Add SelectView::(try_)iter_mut()
• Dialog::{test, info} now accept StyledString as input
• Add missing functions to Checkbox re: enabled state

Bugfixes

• Fix Ctrl-Z binding for ncurses
• Fix potential crash with empty SelectView
• Add toml and markdown features to docs.rs