feat: support server variables

[rwalle61]

For #89

Supercedes #161 and #218. Thanks @JonnySpruce for your work on this!

I think I've overcomplicated your code in order to output these error messages. Perhaps we can simplify it in future :)

[codecov]

Codecov Report

Merging #222 (9941c32) into master (39c5d60) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #222   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        17    +1     
  Lines          373       400   +27     
  Branches        49        56    +7     
=========================================
+ Hits           373       400   +27     

Impacted Files	Coverage Δ
...enapi-validator/lib/classes/AbstractOpenApiSpec.js	100.00% <100.00%> (ø)
.../openapi-validator/lib/classes/AbstractResponse.js	100.00% <100.00%> (ø)
...ages/openapi-validator/lib/classes/OpenApi2Spec.js	100.00% <100.00%> (ø)
...ages/openapi-validator/lib/classes/OpenApi3Spec.js	100.00% <100.00%> (ø)
...penapi-validator/lib/classes/SuperAgentResponse.js	100.00% <100.00%> (ø)
...ckages/openapi-validator/lib/openApiSpecFactory.js	100.00% <100.00%> (ø)
.../openapi-validator/lib/utils/OpenApi3Spec.utils.js	100.00% <100.00%> (ø)
...ckages/openapi-validator/lib/utils/common.utils.js	100.00% <100.00%> (ø)

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[rwalle61]

So I wasn't aware that Travis recently changed their pricing plan such that open source projects no longer have a generous allowance of builds. We seem to have very quickly used up our one-off allowance. Following their FAQ, I've requested more build minutes - hopefully they'll be generous.

If not, we'll have to migrate to another build provider, perhaps GitHub Actions, though I haven't looked into it yet

[rwalle61]

Looks good for merging and publishing after CI is fixed and I've corrected the two test names

[rwalle61]

note: we use variableName here but variable on line 39. I think by definition a variable is a variableName, but in the openapi spec a server variable is a complex object. This is an example of how migrating our code to TypeScript in future will hopefully help clarify it

[rwalle61]

This file could definitely be simpler, but I'm not sure how. Will be very grateful for suggestions! The tricky thing is to provide error messages with server urls that are templated before the path, and concrete after the path, e.g. https://{hostVariable}.com:{portVariable}/defaultValueOfVariableInPath (See the tests)