Template tags should be able to "emit" files in a way Baker can process

[rdmurphy]

I'm thinking of something very, very similar to Rollup's this.emitFile method. Right now all a tag can do is take in a string and return a string, which means if it'd want to, for example, output more than one file (say, an {% image %} tag that creates multiple images for srcset) it would be impossible.