Fix highlighting of long Unicode escape sequences

The CSS specification states that hexadecimal escape sequences up to six
digits long are allowed within both single- and double-quoted strings.
Previously, double-quoted strings allowed up to six decimal digits, but
not hexadecimal. And double-quoted strings didn't allow more than one
character escape sequences.

This change unifies the pattern used for both single- and double-quoted
strings. It also allows both string types to have escape sequences
consisting of a backslash followed by any character or a backslash
followed by one to six hexadecimal digits.
Merge pull request #18 from atom/ld-long-escapes

Fix highlighting of long Unicode escape sequences