Custom args in yml file

Is it possible to pass custom args to a danger script file?
```
with:
args: -myarg ${{ someValue }}
```
So that in the script I can get it like
```
danger(args) {
val myValue = customArgs["myarg"]
}
```

Start by tracing how the YAML `with.args` value reaches the Danger script and how `customArgs` would be exposed. Done means a custom `myarg` value can be passed from the YAML configuration and read by the script as shown, with coverage for the argument-handling path.

Written by the indexing model from the issue text.