Add support for --inject option for the bundle command

Writes bundle to config.js:

```bash
jspm bundle app/main outfile.js --inject

     Building the single-file dependency tree for app/main...
ok   "outfile" added to config bundles.
ok   Built into outfile.js
```

config.js:
```
System.config({
  "paths": {
    "*": "*.js",
    "npm:*": "jspm_packages/npm/*.js",
    "github:*": "jspm_packages/github/*.js"
  },
  "bundles": {
    "outfile": [
      "app/foo",
      "github:components/jquery@2.1.1/jquery.min",
      "github:components/jquery@2.1.1",
      "app/random",
      "app/main"
    ]
  }
});
```

Fixes #69