Support index more than 9

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:
```python
cssify("/li[10]")
# cssify.cssify.XpathException: Invalid or unsupported Xpath: /li[10]
```
Expected behaviour is:
```python
cssify("/li[10]")
# 'li:nth-of-type(10)'
```
