Add --env cli flag (ref #692)

This PR enables adding environments using a new `--env` CLI flag. It will not disable environments specified elsewhere; it only enables additional environments. To guarantee the set of enabled environments matches exactly what is specified via this flag, combine with the `--reset` and `--no-eslintrc` flags. The list of environments following this flag must be comma-separated, as the flag cannot be used more than once.

Example:

```
eslint --env browser,node file.js
```