Support for Bitwarden custom fields in mrc-converter-suite

@leblub

I've updated the converter suite to add these features.

Use the --addfields option to get your custom fields placed into a 1Password section.

Custom Hidden fields will be concealed in 1Password with this option.

Linked fields will show the link's referent field (i.e. Username or Password). Links are essentially a Username or Password alias in Bitwarden, to allow form filling of HTML fields that don't match "username" or "password".

Custom fields of type Text and Boolean were already supported, and --addfields would have placed them into a 1Password section.

Without --addfields, all custom fields will go to the Notes section.

Not that Identity types may be expanded into several additional 1Passwords records (Social Security, Passport, and Drivers License). Without --addfields, the custom fields for the original Identity record will end up in the Notes fields for all the records resulting from expansion. When --addfields is used, only the Identity record will contain the custom fields. This is a simply an implementation limitation and quirk, that is not worth resolving.