Move to plugin

[szszszsz]

@tomholub You mentioned that Nitrokey WebCrypt support in OpenPGP.js could be potentially made as a plugin, but I have not found such system in OpenPGP.js. Can you extend on that thought?
That might be interesting target long-term.

[tomholub]

I don't think OpenPGP.js is ready for such plugins today. I think they will be receptive to a PR which would allow certain methods within the library to be replaceable through configuration, or a similar small change that would allow for development of an OpenPGP.js - nitrokey interface in a separate repository, which could then be used together.