allow whitespace characters to work with replace mode

[J-Fields]

Fixes #767

[JugglerShu]

Can you add test cases to the PR?

[structAnkit]

This works except for three things:

1. It doesn't replace the last character on a line with an endline; it only deletes that character.
2. When "replacing" the last character on a line with an endline, XVim breaks and j will not go to lines below the line whose last character you attempted to replace.
3. When replacing any character but the last one on a line, an endline does replace the character, but it doesn't auto-indent the way you'd expect as you would if you instead do something like x i <Enter>.

I'm not positive how to resolve the issues but would love to see them addressed and implemented! This has been a major pain point of XVim for me.

[elzup]

A problem occur after this action(r[Enter]).
Undo command not working.

[jasonmay]

for r<cr> #951 should hopefully handle that

EDIT: looks like these are trying to accomplish the same things. maybe they should be combined if isWhitespace includes \r? I'm not at an environment to check at the moment.