TextRank keyword extractor

[ChristianMurphy]

TextRank specializes in automated summarization/keyword extraction on natural language.

TextRank could be integrated with retext as a plugin
TextRank Paper: https://web.eecs.umich.edu/~mihalcea/papers/mihalcea.emnlp04.pdf
Some sample implementations that could be used as a starting point:

• https://github.com/dpressel/textrank-js
• https://github.com/bibhuticoder/summaryjs

Previously discussed at retextjs/ideas#5