Add support for aliases

[gavv]

httpexpect is able to print assertion chain path on failure. For example, when you write:

e.GET("/endpoint").
  Expect().
  JSON().Object().Value("foo").
  Array().Element(0).Number()

and assertion fails, you will see:

        expected: value is number
        
        assertion:
          Request("GET").Expect().JSON().Object().Value("foo").Array().
            Element(0).Number()

Often it is convenient to create variables when making assertions:

foo := e.GET("/endpoint").
  Expect().
  JSON().Object().Value("foo").Array()

foo.Element(0).Number()
foo.Element(1).Number()
...

In this case, it would be nice if variables names were reflected in the failure message.

This issue suggest to implement support for aliases, which will work like this:

foo := e.GET("/endpoint").
  Expect().
  JSON().Object().Value("foo").Array().Alias("foo")

foo.Element(0).Number()

        expected: value is number
        
        assertion:
          foo.Element(0).Number()

To make it possible, the following changes are needed:

• Add AssertionContext.AliasedPath. It will be similar to AssertionContext.Path, but will take into account aliases.
• chain.enter() and chain.leave() should update both Path and AliasedPath
• chain.clone() should clone both Path and AliasedPath
• chain will provide new setAlias(name) method that will replace AliasedPath with a new slice with single element name, and keep Path untouched
• To every inspector struct (Array, Object, Number, etc), add Alias() method that accepts a string name and returns receiver; it will just call chain.setAlias.

After this changes, AssertionContext.AliasedPath will automatically contain the value we're interested in. Now we just need to update DefaultFormatter:

• add DisableAliases flag (zero by default), which disables new feature (enabled by default)
• if aliases are enabled, fill FormatData.AssertPath using AliasedPath instead of Path

And that's it.

We also need to add unit tests:

• comprehensive tests for alias support in chain
• short unit test (or maybe new assertion in existing test) for every inspector object (Array, Object, etc)
• e2e test for aliases in reports (see Add end-to-end tests for report formatting #165)

Finally we should add documentation:

• added public methods should be documented
• we should add a section to README that explains the feature

[k4n4ry]

Hi @gavv, can I pick this up?

[gavv]

You're welcome, thanks!