Add emaps--read-keymap to public API

[GuiltyDolphin]

It may be useful to use emaps--read-keymap outside of Emaps, so add it to the public API.

• Support the same arguments as completing-read, bar COLLECTION

  • These arguments will need to be supported by emaps--completing-read-variable too

• Rename emaps--read-keymap to emaps-completing-read-keymap