Add support for documenting partitioning and clustering columns

BigQuery has support for partitioning and clustering columns, and this doesn't appear to be supported in DBML. Would it be possible to add support for these features? I'm thinking it could be implemented in a similar fashion to `indexes` (i.e. hover over the table name to see this information). Example usage:

```
Table table_a {
id string
created_at timestamp

// table_a is partitioned on the created_at column with a granularity of hour
partitions {
created_at [granularity: "hour"]
}

// table_a is is also clustered on the id column
clusters {
id
}
}

Table table_b {
id integer
created_at timestamp

// table_b is partitioned on the id column where values 0-9 are in one partition, 10-19 are in another, etc
partitions {
id [start: 0, end: 100, interval: 10]
}
}
```