Feature request: Lint command

## Summary

It would be great if the CLI had a `lint` command like `@vue/cli`, except it might use `eslint-plugin-gridsome` instead of the one for Vue projects.

### Basic example

```bash
gridsome lint
gridsome lint --fix
```

### Motivation

I always have a linter (namely ESLint) which allows me to ensure my code is readable and consistent which ESLint handles well, even on Vue projects where the `lint` service is handy.
It's also quite time-consuming to manually set up ESLint with the correct plugins and configurations in Gridsome projects and I think it would be even better to have it out-of-the-box (which I'm happy to help with).