Improve mappings setup

- re-map `<CR` via maps, not from the functions. The convention appears
  to be that the `<Plug>FooCR` mappings do not trigger `<CR>`
  themselves.
- Add `<Plug>CursorCrossBS` mapping.
- Add s:setup_map to setup both CR and BS mappings: this improves on
  the version based on endwise from the previous commit:
  - Use <unique>, catch any error and provide help.
  - Handles g:cursorcross_no_map_CR and g:cursorcross_no_map_BS to
    skip mappings.
- Use `<unique>` also with mapping for settings: this will not overwrite
  them and provide a hint to the user.

Ref: https://github.com/mtth/cursorcross.vim/issues/2