Improve Marks mode UX

Improve Marks mode UX

- Return to Normal mode if a mark key hasn't been pressed within
  `options.timeout` milliseconds.

- Let `<escape>` return to Normal mode instead of creating or going to a
  mark named `<escape>`. Previously, if the user pressed `<escape>` as
  an attempt to exit Marks mode after accidentally pressing `m`, that
  would indeed exit Marks mode, but it would also set the `<escape>`
  mark. That meant that after that point, the user couldn't exit Marks
  mode using `<escape>` after having pressed `` ` `` without the side
  effect of scrolling to the `<escape>` mark.

- Marks mode has been hidden from the help dialog, in order not to
  clutter it with unnecessary information. While at it, Find mode was
  removed from the help dialog as well, for the same reason. Both modes
  are still available in VimFx's settings page in the Add-ons Manager,
  though.

Fixes #750.