Fix history cycling

Previously, the displayed command didn't change when switching cycling
direction. This resolves the issue.

imrsh_move: handle UTF-8 properly

Several other functions remain to be addressed

Don't append repeated commands to history

build: add subproject fallback for mrsh dependency

Makes it easier to debug both imrsh and mrsh at the same time.

Implement simple cycling through history

history: use doubly linked list

Write command history to file

add c-l (clear)

README.md: add ticket tracker link

add imrsh_swap

Call interactive_finish after interactive_next

This resets all of our terminal crap so that any child programs can do
their thing with a sane terminal.

add M-f / C-Right bindings

add M-b / C-Left bindings

add imrsh_delete_all

add imrsh_backspace_all

add imrsh_backspace_word

README.md: add rebindable keys to goals

add imrsh_delete

We will not be using /dev/tty

This removes the comment which wondered if we might.

Set pens explicitly before use