Upgrade link checking to include csv and other text files

[wwarriner]

The link checker here will check plaintext files: lycheeverse/lychee#59

There is no install available directly via conda-forge nor pypi. The binaries will have to be downloaded from github, instead. This should be possible using pip from within our conda environment, but it isn't clear to me, yet, how to do it with a pre-built binary release.