Preliminary support for 'R' mode

Hey, this is a preliminary support for REPLACE mode. I moved the "oneChar" boolean to a new ReplaceEvaluator. It works but there are a couple of things missing:

 * 'u' should undo the entire block not char by char.
 * It should never replacing the newline (end of line). This is an existing bug on the 'r' mode as well.
 * When entering "backspace" on R mode, vim shows the previous char instead.

Other than, I think it's a good idea to create a new evaluator for the Replace Mode instead of having a bool on insert. What do you guys think?