Allow custom xml and json content types

[goughy000]

As per RFC7303 and similar for json this PR allows usage of headers such as

application/xml
application/xml;charset=UTF-8
application/vnd.com.whatever+xml
application/json
application/json;charset=UTF-8
application/vnd.com.whatever+json

to fall into the JSON and XML matching methods

[codecov-io]

Codecov Report

Merging #76 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
+ Coverage   98.95%   98.96%   +<.01%     
==========================================
  Files          35       35              
  Lines        1057     1062       +5     
  Branches      175      177       +2     
==========================================
+ Hits         1046     1051       +5     
  Misses          4        4              
  Partials        7        7

Impacted Files	Coverage Δ
.../github/azagniotov/stubby4j/stubs/StubMatcher.java	98.83% <100%> (+0.07%)	⬆️

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[azagniotov]

Thanks! I will look into this ASAP!

[goughy000]

Thanks! Although thinking about it maybe it would be better to just match like \/(?:.*\+)?(\w*);? and then check if the group is "json" or "xml"
What do you think?