jsonmunge should use Mustache templates rather than Go templates

[rsdoiel]

The go templates are just not widely understood outside of Go developers. Mustache is common in many languages an there is a good Go Mustache package. I need to swap out the template engine for Mustache, maybe leave the go one as an option though I don't think anyone actually used it besides me.

If I do this I'll need to bump the semver from 1.2 to 1.3.

[rsdoiel]

Better yet, use Handlebars templates. It's an easier transition but maintains the spirit of Mustache.