Add optional ampersand on variable names.

I'm adding an optional ampersand to property references in SkyQL. It's meant to avoid name clashes between SkyQL reserved words and people's variable names. It won't affect existing queries.

For example, if you have a property named `EVENT` then you can still query it even though that's a reserved word:

```skyql
SELECT sum(@EVENT) GROUP BY @foo, @bar
```

You could also make the `foo` and `bar` bare words:

```skyql
SELECT sum(@EVENT) GROUP BY foo, bar
```

Safety first!

/cc: @boourns @orenmazor @snormore