Add misc.Macro actor

Replaces #149 
Closes #142 
Two things ..

1. I think at this point its critical we build a URL parser for all file references that supports some basic protocols: `https://` `http://` `file://` at least.
2. When parsing out `file://`, we should have a predictable behavior if you're not in the same directory as the initial `--json <file>` that was passed in. I'm not sure if we should just treat all paths as relative to the `pid`, or whether they should be relative to the initial script?