Optional minification & uglification

[vbwx]

Related to #30, I think it would make sense to prevent minification of requested files in a development environment (and to always minify them in a production environment). That would make debugging a lot easier when building a site. Alternatively, Harp could offer a command line switch to disable this behavior.

[jimjkelly]

To add on to this - in hacking in ES6 support, adding imports in results in a lot of duplicate code. By default the settings passed to Uglify turn off mangling and compression. It'd be nice to be able to turn these on somehow, as it results in a significantly smaller payload for me. I'd be open to submitting a PR for this, though Terraform has largely been kinda zero configuration (a strength!) - so I'm not sure there's an avenue to do that. Suggestions? Is this unwanted?