add .where & .orWhere @ CreateIndexBuilder.

[igalklebanov]

closes #302.

Adds the ability to create partial indexes in postgres / sqlite.

• only columns known in compile-time (used in .column/.columns) provide autocompletion,
  unknown columns require sql template tag to be referred to.
• parameter types are unknown.
• parameters are not allowed (engine throws), so they're turned to literals before parsing.