Allow explicitly named primary keys, foreign keys, and indices

a `constraintName` option on `@Index`, `@PrimaryGeneratedColumn`, and `@JoinColumn` would be nice.

example:
```
@Entity()
class Post {
  @PrimaryGeneratedColumn({ 
    constraintName: 'pk_post_id' 
  })
  id: number

  @ManyToOne(author => Author, author => author.posts)
  @JoinColumn({ 
    constraintName: 'fk_post_author_id'
  })
  author: Author
}
```