Add support for latest RegExp JavaScript features

[ehmicky]

It would be nice to support the latest RegExp JavaScript features:

• \p and \P: Unicode property escapes
• (?<group>) and \k<group>: Named groups (feat: add support for named capture groups #43)
• (?<=) and (?<!): Negative lookbehind assertions

They are all now part of the EcmaScript standard. Node 9 does not support them but Node 10 will.

Thanks for this project, it's really useful.

[ehmicky]

Node 10 is now out and supports Unicode property escapes and named groups. Negative lookbehind assertions were actually already supported in Node 9.