Compute prompt completion only from characters left of the cursor

If I type

	:echo -mx

I get no completions, even when I move the cursor on the x.
If I replace the x  with a k, I get a completion "-markup".
The odd thing is that if I accept the completion while the cursor is
on the k, then the commandline will be

	:echo markupk

Evidently, the characters under cursor (x/k) influence the completion
(actually all letters right of the cursor do), but they are not
incorporated in the final result, which is weird.

I think there are two consistent behaviors:
1. Compute completions only from characters left of the cursor. We already
   do this in insert mode completion, and when completing the command name
   in a prompt.
2. Compute completions from the whole token, and when accepting a completion,
   replace the whole token.

Most readline-style completion systems out there implement 1. A
notable exception is fish's tab-completion. I think we should stick
to 1 because it's more predictable and familiar. Do that.

This allows us to get rid of a special case for completing command
names, because the new behavior subsumes it.

In fact, I think this would allow us to get rid of most "pos_in_token"
or "cursor_pos" completer parameters. I believe the only place where we
it's actually different from the end of the query is in "shell-script"
completers, where we expose "$kak_pos_in_token". I think we can still
remove that parameter and just cut the commandline at the cursor
position before passing it to a "shell-script" completer. Then we
also don't need "$kak_token_to_complete" (but we can still keep
expose them for backwards compatibility).

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