[Pitch] Adding HTML coverage support

[dschaefer2 (Doug Schaefer)]

@bkhouri

Do you have a pany preferences between (1) the ability of specify the new option multiple times (e.g.: --formats json,html or --format json --format html) or (2) have the ability to generate a coverage report from a previous run?

We have similar options for swift package describe

USAGE: swift package describe [--type <type>]

OPTIONS:
  --type <type>           Set the output format. (values: json, text, mermaid; default: text)

Though I don’t really like the term type.