Improve vocabulary support

[justin-tay]

Closes #753

This improves the support for the $vocabulary keyword used in a meta schema.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (51fd82b) 78.98% compared to head (a59ad6d) 78.99%.

Files	Patch %	Lines
src/main/java/com/networknt/schema/Vocabulary.java	68.75%	10 Missing ⚠️
.../java/com/networknt/schema/UnionTypeValidator.java	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##             master     #953   +/-   ##
=========================================
  Coverage     78.98%   78.99%           
- Complexity     1717     1724    +7     
=========================================
  Files           157      158    +1     
  Lines          5715     5750   +35     
  Branches       1079     1086    +7     
=========================================
+ Hits           4514     4542   +28     
- Misses          802      809    +7     
  Partials        399      399           

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