Declarative support?

This is feedback based on using brigade side-by-side with azure pipelines for ~6 months for Porter.

There has been quite a bit of a steep learning curve in figuring out both how brigade's libraries, javascript, GitHub actions and events. Our brigade file has always been an exercise in copy/pasting from a working brigade.js file in someone else's project, or picking @vdice's brain. 😀

It would be helpful if there was a simplified version of brigade's agent that was optimized for helping me just run makefile targets for my CI. Really that's where I keep all of my logic, in my makefile and all I want to do is call a few different variations of targets for a branch or tag.

Right now our brigade.js file is [pretty big](https://github.com/deislabs/porter/blob/master/build/brigade.js), but the entire thing boils down to this (or some subset for a PR):

```
make test-unit test-integration test-cli publish publish-examples
```

Some of the brigade.js file has more logic than that, but really that logic should be in a makefile target, it's not only because at this point we are relying on things already being installed on an azure pipelines agent, which is cheating. 😁

But if we could distill it down or make a lot of the noise in that file go way, and be able to use a declarative language with a documented schema, that would have made me feel a lot more comfortable picking up brigade and managing it myself.

I'm not trying to change how brigade works. Just saying that if we could add a mode on top of brigade that let me work more like I'm used to that maps to normal brigade under the hood, that would be interesting to look into.