Improve multibyte handling especially for g:EasyMotion_key !

Relate: #16
Improve readability and add NOTE for multibyte handling
Perform refactoring of marker substitution section
Fix multibyte handling for EOL and bland line
Unify the way to get marker_char from marker_chars
Fix to get target_char correctly

Get from marker, modified line, instead of original target line.
Escape '&' for substitution
Merge branch 'handle/multibytetargetkey' into master

close #16