Interactive image insertion and editing

Interactive image insertion and editing

* Image insertion and editing has been consolidated into one command,
  `markdown-insert-image`, bound to `C-c C-i`.  As such, the previous
  separate image insertion keybindings have been removed: `C-c C-i i`
  and `C-c C-i I`.

* The old inline image toggling command `C-c C-i C-t` has been removed
  and replaced `C-c C-x C-i` in order to allow for the new interactive
  image insertion command at `C-c C-i`.  Toggling keybindings are
  currently being grouped under `C-c C-x`.

* The non-interactive image insertion commands have been refactored to
  mirror the corresponding link insertion commands.
  `markdown-insert-image` (for inline images) has been renamed
  `markdown-insert-inline-image` and it now takes three
  arguments (previously one optional argument).
  `markdown-insert-reference-image` now takes four
  arguments (previously none).