Use Consolidate rather than jade

[colophonemes]

I've refactored to use Consolidate.js. The plugin now takes an engine and ext parameter (defaulting to jade and .jade respectively.

This is important for my use case, because it means I can use the same version of Jade/Pug for both metalsmith-shortcodes and metalsmith-layouts, but presumably this is useful for those using different templating languages.

This also shifts Jade to devDependencies (so the tests still run), so if you're using this in another project you need to ensure you've got the engine installed

[ericgj]

Thanks for these updates. Since consolidate has an async interface, I think it makes more sense to use promises instead of async.each and a hacky workaround in the consolidate callback. (Arguably it would have been better to use promises from the beginning, but I wasn't as familiar with them at the time). Not sure when I'll have time to do that, but feel free to give it a shot if you want.