Add support for html5ever as an optional HTML parser

[philss]

We made html5ever and Floki work together through ex_html5ever thanks to @hansihe and the Rustler team! 👏

Using ex_html5ever can be very useful those who need a more accurate parsing of the HTML. It solves the issues #50 and #75, for example.

Since ex_html5ever relies on a Rust NIF, it requires the user to have Rust installed.

Also I ran some benchmarking that shows the ex_html5ever version is faster from the average to big HTML files. ⚡️

UPDATE: the html5ever Elixir NIF was named to "html5ever" on Hex.pm, and the repository is now https://github.com/hansihe/html5ever_elixir

[philss]

Closed by #90 .

It was released in version 0.14.0.

Thank you @hansihe and @aphillipo!