Add Flesch–Kincaid readability test score to blog posts

Along with word count, char count, and reading time, include this reading score.

http://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_test

designed to indicate comprehension difficulty when reading a passage of contemporary academic English. There are two tests, the Flesch Reading Ease, and the Flesch–Kincaid Grade Level. Although they use the same core measures (word length and sentence length), they have different weighting factors.

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