Validation options: source vs scheme vs format

[Andrew Scholer]

I would like to be able to run a terse version of the validation report using the dev schema. Currently that is not an option as -M assumes a single value.

I propose making that -M flag accept multiple values to specify the three concepts: where to run; which schema to use; how to format the output.

So you might specify:

... -V -M dev terse
or
... -V -M server dev verbose

This PR has an implementation. It keeps the existing options as valid (like local-dev), but supports 

https://github.com/PreTeXtBook/pretext/pull/3146