Add config to override the default register used

[jonasws]

This has frustrated me for a while: At many keyboards the mapping to access the " key is quite cumbersome, in that it is not necessarily caught by the Atom event system (I discovered this by playing around with the Keybinding resolver). This makes it near impossible to access the default register as it is in the current implementation. This is especially true on many international keyboards (me myself having a Norwegian keyboard layout).

Also, I feel it is handy to be able to map the default register to a different key, so I created this PR. It adds a config field that is read from when populating the settings module, and defaults to the " register, so it is all backwards compatible.

[jonasws]

Note that this feature adds no specs, so it does pass all the checks currently passing on the master branch at the time of this PR.

[v3ss0n]

no activity for many months now it seems to be dead.