Vi additions and fixes by @zim0369

[sholderbach]

Rebased the commits from #406

• vim's 'S' binding
• copy char while deleting in vi mode
  • Fixes the behavior of x so the removed char/grapheme can be pasted with p
• vim's 's' binding
• Minor clarifications
  • Fix doccomments and clarify an enum variant's name

Closes #406