Added ability to customise the set of references that are resolved

[stueynz]

Sometimes you don't want all the $ref references to be resolved; added a new options to control which references are dereferenced. e.g.

const options = {
  dereference: {
    circular: true,

    isRefResolved: function(value) {

      // don't resolve where $ref contains the word 'gender'
      return value.$ref.match(/\/gender/g) === null;
    }
  }

[coveralls]

Pull Request Test Coverage Report for Build 352

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 93.891%

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Totals
Change from base Build 348:	0.04%
Covered Lines:	675
Relevant Lines:	708

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💛 - Coveralls

[JamesMessinger]

Seems like this would be a good opportunity to use file-path-filter, which would let people filter $refs based on globs, regular expressions, custom filter functions, or any combination of them.

Also, it seems like this should be a resolve option, not a dereference option. So something like this:

const options = {
  resolve: {
    // don't resolve where $ref contains the word 'gender'
    filter: "!**/gender/**"
  }

[philsturgeon]

That was a cool suggestion James. If anyone would like to pick that up, please make a new PR!