Add support for Index parsing

[julioz]

So far, dbml-parser doesn't properly handle indexes defined in DBML tables, as such:

Table bookings {
  id integer
  country varchar
  booking_date date
  created_at timestamp

  indexes {
      (id, country) [pk] // composite primary key
      created_at [note: 'Date']
      booking_date
      (country, booking_date) [unique]
      booking_date [type: hash]
      (`id*2`)
      (`id*3`,`getdate()`)
      (`id*3`,id)
  }
}

There are 3 types of index definitions:

Index with single field (with index name): CREATE INDEX on users (created_at)
Index with multiple fields (composite index): CREATE INDEX on users (created_at, country)
Index with an expression: CREATE INDEX ON films ( first_name + last_name )
(bonus) Composite index with expression: CREATE INDEX ON users ( country, (lower(name)) )