v1.9.0

This is a major feature release with BREAKING CHANGES (see Migration section for details).
Big thanks to @ricardojmendez for most of the work for this release!

• BREAKING: update-item args have changed (:update-map is now optional) [@ricardojmendez] [1]
• BREAKING: update-table args have changed (:throughput is now optional) [@ricardojmendez] [2]
• New: put-item, update-item support for expressions [@LeonardoBorges #73]
• New: get-item support for :proj-expr, :expr-names [@ricardojmendez]
• New: query support for :filter-expr, :proj-expr [@ricardojmendez]
• New: update-table support for index modification [@ricardojmendez]
• New: scan support for indexes, :expr-attr-names, :proj-expr [@ricardojmendez]
• New: delete-item support for expressions [@ricardojmendez]
• New: scan support for support filter-expr, expr-attr-vals [@ricardojmendez #90]
• New: scan support for consistent reads [@ricardojmendez #92]
• Change: update-table now returns a future instead of a promise (allows exceptions to rethrow)
• Change: implementation details now marked as private
• Fix: remove-empty-attr-vals vs blank strings [@crough #72]