[WIP] Support *:nth-of-type through XPath extension function

[redapple]

Very rough start at supporting *:nth-of-type(an+b)

scrapy/cssselect#4 looks impossible to translate in pure XPath 1.0, but is doable with custom XPath functions.

To do:

• test more an+b cases
• test *:first-of-type, *:last-of-type, *:only-of-type
• test with namespaces
• refactor to use a namespace prefix and a seperate module for custom XPath functions (custom xpath support #13 (comment) and subsequent comment)

[codecov-io]

Current coverage is 96.10% (diff: 68.96%)

Merging #73 into master will decrease coverage by 3.89%

@@           master        #73   diff @@
========================================
  Files           4          4          
  Lines         203        231    +28   
  Methods         0          0          
  Messages        0          0          
  Branches       35         39     +4   
========================================
+ Hits          203        222    +19   
- Misses          0          6     +6   
- Partials        0          3     +3   

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[Gallaecio]

Should we close this?

The reference to this pull request remain in #4, so anyone looking to work on this will not need to start from scratch.