Running tests could be automatic during development

Like Guard with Ruby, we could use Sentry with Crystal to run specs every time one changes.

https://github.com/samueleaton/sentry

We could have a Makefile that allows us to build the binary :

```console
$ make build
```

Or release :

```console
$ make release
```

Or run tests :

```console
$ make test
```

Or watch folders and run tests

```console
$ make sentry
```

