.md file in data dir causes hugo to fail

[mxmason]

hugo version: 0.46

Steps to reproduce:

• Update hugo (to at least 0.4.x I assume)
• run cupper's npm run build

Expected results:

• Happy, functional cupper build

Actual results:

• Failed to read data from cupper/README_BLANK.md/README_BLANK.md: Data not supported for extension 'md'

Per a Hugo forum discussion, it appears that:

• only .csv and .json files are allowed in the data/ directory
• previous versions evidently ignored errors related to this

Placing README_BLANK in another directory seems to work. e.g., create a clean dir, move README_BLANK, update the clean.js script appropriately.

(possibly related to #35)