Implement -0 (--zero)

This makes peco accept lines with NUL(\0) characters.
Anything before the NUL is used as the string to be displayed
in the peco view AND used for matching.
Anything after the NUL is used as the output when peco is done

Should fix #76
I still don't want to drop go1.1
Fix off-by-one

https://github.com/lestrrat/peco/commit/6f4e89e78792c209b16c829a4c26ca6fa64cf5e5#commitcomment-6706530
Remove stray debug statement
Merge branch 'master' into make-some-pizza

Conflicts:
	matchers.go
Revemo -0. Now it's just --zero
Merge branch 'master' into make-some-pizza

Conflicts:
	ctx.go
	keymap.go
	matchers.go
Rename --zero to --null
Note that --null is experimental in the help message, too
Merge pull request #77 from lestrrat/make-some-pizza

Implement --null