Support index more than 9

[GeorgeGorbanev]

Sup! Thank you for lib, but now we can't use it cause of bug with indexes. When I put xpath with index more than 9:

cssify("/li[10]")
# cssify.cssify.XpathException: Invalid or unsupported Xpath: /li[10]

Expected behaviour is:

cssify("/li[10]")
# 'li:nth-of-type(10)'

[sosie-js]

Hello @GeorgeGorbanev ,

This is due to regexp limitation in validation_re (line 24 of cssify.py), it should be \d+ like this
"([(?P\d+)])?"

@santiycr can imho fix this easily.

[santiycr]

@sosie-js thanks for jumping in! I'd be happy to merge if you can send a PR