Make tab completion circular

In general, liner tries to follow the keyboard shortcuts and conventions
of `bash`s defaults. In this case, however, I prefer inline completion
to non-completion (and listing of possible alternatives).

* bash does nothing on the first tab, and prints a list of completions
  on the 2nd tab.
* tcsh prints a list of completions on the first tab.
* cmd.exe does circular completion in-line
* zsh does circular completion in-line, and prints alternatives below
  the line
* liner did completion once, then beeped.

liner shouldn't make up new conventions. Since I would like liner to
stay on one line, printing a list of alternatives is out. Therefore,
liner will behave like cmd.exe in this instance.

Fixes #10 and #11