Script_Extensions value "Unknown" is not covered

[gibson042]

Per UAX #24,

the [Script_Extensions] scx value for unassigned, private use, and non-character code points is the set { Unknown }

But this library does not include that value in its "Script_Extensions" data or the expected Script_Extensions/Unknown.js file.

[JLHwung]

Fixed in 9dd49cd.

[mathiasbynens]

Context: the core of the problem was that the Unknown value was not supported in node-unicode-data. @JLHwung fixed that here: node-unicode/node-unicode-data#93