Configurable proxy blacklist

[lucasfcosta]

This aims to close #765.

As brilliantly suggested by @meeber this adds a new Array containing keys which should be ignored when checking for non-existing properties on an assertion before throwing an error.

This PR contains:

• An awesome and difficult replacement of a backtick which should have been an '
• A new setting on chai.config called proxyExcludedKeys which receives an array of Strings representing the properties which should be ignored by the proxy on environments that support it
• Docs explaining how it works, default values and other peculiarities
• Tests checking:
  • Default values
  • If it does not throw an error on non existing then and inspect properties
  • If it throws an error on non existing then and inspect properties if they get removed from the proxyExcludedKeys array on environments that do support proxies

Let me know if I missed anything.

[keithamus]

This LGTM (looks great to me). @meeber thoughts?

[meeber]

Couple nits:

• Line 78: Can remove "non-existing" since it's redundant with "if they do not exist on the assertion" later on in the same sentence.
• Line 80: "running in" instead of "running into".

[meeber]

@lucasfcosta Awesome work, thanks for doing this! I just had a few nits related to comments. I really like how you handled the changes, particularly the tests with the helper function. You've done the world a great service by fixing that errant backtick.

[lucasfcosta]

Thanks for the help @meeber, since I'm not a native English speaker I make this kind of mistakes sometimes.
Now the comments really seem less redundant and more informative, thank you again for the input 😄

Let me know if I forgot anything.

[meeber]

The part about Symbol properties and Symbol.toStringTag can remain in proxify.js because that's where the type check is; it's different than the configurable excluded keys. A new note needs to be added here about .inspect; it's ignored by default because that property is read by util.inspect, for example when using console.log on the assertion object.

[meeber]

@lucasfcosta I'm jealous of your ability to speak multiple languages!

I added a follow-up note above about the comments.

[lucasfcosta]

@meeber thanks for noticing that!
I have made the fixes you suggested and I also read the inspect utility and found some other minor typos, so I also fixed them.

[meeber]

LGTM!