add option for pasting by default from systemclipboard

[ianhi]

allowing for yanking to the system clipboard register to be the default

[ianhi]

this might also require overriding the codemirror vim paste to change the default paste register

[ianhi]

which is substantially larger than yank
https://github.com/codemirror/CodeMirror/blob/b2d26b4ccb1d0994ae84d18ad8b84018de176da9/keymap/vim.js#L2563

[ianhi]

And will require re-implementing three vim utility functions that aren't exposed:

1. getSelectedAreaRange
2. copyCursor
3. findFirstNonWhiteSpaceCharacter

[ianhi]

For pasting within jupyterlab reimplementing isn't necessary because the unnamed register will be used by paste. So if I take unamedplus to mean fill both the + and unnamed register then it will get most of the functionality

[filyp]

+1 pasting would be very useful