indent / lint all listing during build

[duncdrum]

Is your feature request related to a problem? Please describe.
Currently listings ending in .xml are not linted in a uniform manner, which can make them harder to read. This is due to the individual settings that different contributors have locally when creating PRs.

Describe the solution you'd like
Lint/ Beautify all listings during build so that we get a more uniform appearance.

Describe alternatives you've considered
Prescribe via author-reference but that would just increase the work for reviewers.

[duncdrum]

apparently there is a bug with repeated runs of the linter see vkiryukhin/pretty-data#11 although we are minifying first, i see the same growth in empty lines, not good.

https://github.com/t1st3/gulp-muxml