up start improvements

Just an issue for #239, the gist of it is to provide local build/clean hooks so that `up start` starts the proxy and runs `node app.js`, or `go run main.go` etc for you.

The alternative is to replace `up start` with a less magical `up proxy PORT`, so instead of running:

```
$ up start
```

You'd run something like the following, where `5000` is the port `app.js` is listening on.

```
$ node app.js
$ up proxy 5000
```

and visit the proxy's port instead of your app's. This might be a bit more confusing, but you also need to be aware of less magic which could be potentially confusing as well if you start deviating from the conventions (of `./app.js`, and `./main.go` etc), because then you'd need something like:

```json
{
"proxy": {
"local_command": "go run cmd/api/main.go"
}
}
```

Opinions?

[](https://m131jyck4m.execute-api.us-west-2.amazonaws.com/prod/poll/01BSA6HHNAP6SVQ3QNA5H4YM6Y/up%20proxy/vote)
[](https://m131jyck4m.execute-api.us-west-2.amazonaws.com/prod/poll/01BSA6HHNAP6SVQ3QNA5H4YM6Y/up%20start/vote)

---

- [ ] allow local overrides of hooks etc
- [ ] add inferred hooks for Go et al
- [ ] document