[MRG+1] Add a css2xpath util function available at the top-level

[eliasdorneles]

I've found useful a small util function to get a CSS query translated to XPath, for cases when I'm using an API that only accepts XPath but I want to use CSS at least for parts of an expression.

Here are some usage examples from recent code that might illustrate the usefulness:

• css2xpath('.profileTopData') + '//h3
• next_page_xpath = css2xpath('.paginationBar a') + '[./span[contains(@Class, "next")]]/@href'

Does this look good to you?

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #64 into master will not change coverage

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