Default to previous search expression for :%s.

In vim, :%s//<replace exp>/ should replace the previous search
expression (that we already store to use with "n").

Also cleans up some of the code here.

Fixes #676.
temp
Fix "c" behavior with newlines.

LineWise c should not delete last newline. Fix by adding newline and
then going back one character.

Fixes #673 and #674.
Persist current regex across :reload.

So that you don't lose your search highlighting on :reload.
Add persistentSearch configuration variable.

Fixes #685 by making persistent search opt-out. Variable defaults to
True, set to False and Yi will ignore the regex when loading
persistent state.