How the input system for vim_mode should work.
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- insert mode mappings
- normal mode mappings
- timeout for ambiguous mappings
  - but support for no timeout for some mappings (like Ctrl-R in insert mode)
- maybe 'timeout' and 'ttimeout' settings (and 'timeoutlen' and 'ttimeoutlen')
- partial commands/mappings (like :map a :b a which inserts ex mode and
  displays :b a there without running any command)
- support for mappings entering insert mode: :map gX itexthere<esc> (also
  without esc!)
- . for all (most) commands
- more general command order: 3"ap doesn't work but "a3p does
- arbitrary mappings, like 3dwjjihi<esc>10G (at the moment even :map Y y$
  doesn't work)
- support to disable mappings temporarily (:set paste, 'pastetoggle')
- (better) support multiple bindings for the same action (especially <BS> and
  <C-H>)
- unmapping of internal mappings (for example <C-C> to <Nop> as it's also used
  for colors in irssi)