Added the ability to supply config keys on mount

Should be especially useful for augeas, e.g.: `kdb mount /etc/hosts system/hosts augeas lens=Hosts.lns`

As always: my c++ knowledge is limited, so please be strict in your review :)
There are numerios ways to do it without heap allocation. In this case I would even completely split the interactive and the non-interactive implementation with their only commonly used code (the 4 lines appending a key) factored out in a function. The function is too long anyway.