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 :)
At least in appendPlugins function this variable still indexes a plugin (because in case of a config it is incremented twice). However, I renamed the parameter of readPluginConfig to current_token for more clearness.