[aggregate] form an openly available corpus for testing

[Nytelife26]

Relevant issues

Supersedes #141, #182, #183.

Brief

A long time ago, proselint had its own corpus for benchmarks and calculating the lint score. It had to be removed, due to copyright issues and lacking distribution rights. It would be great, although not essential, to form an open-source corpus.

[Nytelife26]

While distribution in the repository itself may be impractical, there are public domain corpora we can use.

• Standardized Project Gutenberg Corpus
• Harvard Library Public Domain Corpus