Add a `source` attribute to MixPanel events
Add a `source` attribute to MixPanel events

Use the `User-Agent` header to distinguish CLI and the web.

quiltdata/quilt-compiler,quiltdata/quilt-compiler,quiltdata/quilt,quiltdata/quilt,quiltdata/quilt,quiltdata/quilt,quiltdata/quilt-compiler,quiltdata/quilt-compiler,quiltdata/quilt