Treat combined characters as paired characters
:memo: Mention combined characters
Add combined characters to spec
:memo: Mention combined characters in spec description
Stored paired characters in width cache

Previously characters were treated individually even when multiple
characters would render as a single character on screen.
:racehorse: Only use String::substr for pair chars
:lipstick:
Access value on token
Add spec for cursor position after pair char
Merge pull request #3953 from atom/ks-support-combining-characters

Support unicode combining characters