Support using registers with multiple selections

[ulyssa]

When using multiple selections to yank, delete or paste in Kakoune, the contents are kept as a vector of each selection's text in the register. Kakoune then has multiple keybindbings for pasting these:

• p/P/R which pastes after/before/replaces each selection with the matching indexed contents from the register; if there are fewer selections than in the register, then the extra content doesn't get pasted; if there are more selections than in the register, then the last selection from the register gets reused.
• <a-p>/<a-P>/<a-R> which pastes after/before/replaces every selection with every one of the contents from the register

This will need to be added for #32.