Add ability to specify register when yanking

[jacktose]

NERDCommenterYank (\cy) yanks to the default unnamed register ". Like all other yanking, this is a good default, this gives the user an option to choose another register.
One use case is creating a “comment and duplicate” function/mapping without clobbering the default register, as discussed in #435.
Closes #543

Goals

• ✅ ["x][count]\cy - Support standard register specification for NERDCommenterYank mapping, as with ["x]yy and friends.
• ❌ ["x]<plug>NERDCommenterYank, <plug>NERDCommenterYank({register}), or similar way to specify a register when invoking this way (e.g. in a user-defined function or mapping).
  • I'm not sure if this is possible. IMO it's not critical because it can be accomplished by remapping to the default maps: nmap \abc "x\cy
• ❌ NERDComment({mode}, {type}, [{register}]) - Specify register when type=Yank.
  • I'm very reluctant to mess with the main function signature.
• ✅ let g:NERDDefaultRegister = x - Set default register used by NERDCommenterYank.
  • This works. It defaults to " (so no behavior change by default) and is overridden when specifying a register with "x.
• ✅ Update documentation

Validation

This works for me in vim 9.1.
I'm brand new to vim help files, and not much better at vimscript, so additional eyes and feedback are welcome.

[jacktose]

@alerque Could you take a look?