Render markdown in `covidcast_epidata` output

[nmdefries]

When using the obj output by covidcast_epidata, text is shown with markdown unrendered. Can we render it to make messages look nicer and, for links, easier to interact with? e.g. "output$sources$hospital-admissions" looks like

[1] "Information about hospital admissions, provided to us by health system partners. Using inpatient claim counts, we estimate the percentage of new hospital admissions with a COVID-associated diagnosis code in a given location, on a given day.\n\nSee also our Health & Human Services data source for official COVID hospitalization reporting from the US Department of Health & Human Services."