Read IntChron's export formats

[joeroe]

As well as JSON, IntChron's API can also export data as (from https://intchron.org/) html, csv, txt (a tsv), oxcal, and bib. It would be nice to provide functions for reading this data into R, either directly from IntChron or via a downloaded file.

• HTML: ignore
• csv & tsv: could have straightforward read_ functions with a bit of extra handling for the formatting idiosyncracies
• oxcal: since these generally contain less data than the other formats, I'm not sure it's useful to extract it. That functionality would also overlap with stratigraphr and oxcAAR.
• bib: should be integrated into a more comprehensive workflow for managing references, see Retrieve bibliographic data #7

[joeroe]

CSV and TSV can now be read with intchron_read_delim(); OxCal we will leave for the time being; BibTeX will be handled in #7.