mapping workflow, first attempt

This is a rough draft of a mapping workflow.

The idea is that when I am writing a package, say `thispackage`, I may want to have custom maps set up within that package that will transfer when it is published.

The way we can handle this is with the `package.json` map property, which is supported by jspm:

```json
{
  "map": {
    "custom-path": "value"
  }
}
```

This PR propogates the above map configuration into the configuration file, supporting syncing between the package.json and config file values.