Fixes #13. Support creating different null objects impls in macro.

[limansky]

No description provided.

[codecov-io]

Codecov Report

Merging #14 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   95.86%   95.91%   +0.05%     
==========================================
  Files          13       13              
  Lines         145      147       +2     
  Branches        5        4       -1     
==========================================
+ Hits          139      141       +2     
  Misses          6        6

Impacted Files	Coverage Δ
...ala/com/github/limansky/mongoquery/core/BSON.scala	100% <ø> (ø)	⬆️
...hub/limansky/mongoquery/core/MongoQueryMacro.scala	85.71% <ø> (ø)	⬆️
...mansky/mongoquery/core/bsonparser/BSONTokens.scala	100% <100%> (ø)	⬆️
...github/limansky/mongoquery/casbah/BSONParser.scala	100% <100%> (ø)	⬆️
...b/limansky/mongoquery/core/bsonparser/Parser.scala	100% <100%> (ø)	⬆️
...thub/limansky/mongoquery/reactive/BSONParser.scala	88.88% <100%> (ø)	⬆️
...ub/limansky/mongoquery/core/MongoQueryParser.scala	100% <100%> (ø)	⬆️

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[kell18]

Cool! There is also some more info to original issue #13, I will check it out in a few days