enable adding key to "previousPositionRegisters"

[david206]

To support "jump back to the position before the previous jump" on other languages keyboard,
there is a need to add another keys to previousPositionRegisters in addition to [ "", "'" ]`

[smblott-github]

One idea that was mooted previously for this was to take whatever key was mapped to activate marks (e.g. ') and have two of them consecutively mean jump-to-previous-position.

So, if the activating key was #, then ## would jump to the previous position.