Add ClientOptions to support withCredentials and request debugging

[imalsogreg]

Add a new ClientOptions type and clientWithOpts functions.

The options support user's ability to set the withCredentials bit in xhr requests, and to choose a request debugging function that runs in JSM.

Addresses #46 and #45

RFC @3noch and @NorfairKing what do you think?

[NorfairKing]

I'm happy-ish with this.
I may not know enough about CORS, but don't you ALWAYS want optsWithCredentials = True when using any kind of auth? I'd prefer not to have to think about this myself...

If not, then this is certainly fine.

[3noch]

@NorfairKing You'd only want withCredentials = True if you're actually doing CORS. If you're using auth on your own domain then you don't need it, which is probably the most common case.

[3noch]

@imalsogreg Thanks so much for tackling this! I like your approach. I have one more hope for this feature: I'd like to be able to arbitrarily modify the request before it gets sent (for example, setting custom headers). Would it be possible to instead provide some arbitrary Req -> Req function that can do that (including setting withCredentials)? That would solve both of those cases. If the Req -> Req was Req -> JSM Req, then it would tackle all three needs in one swoop.

[NorfairKing]

@3noch Thanks for the clarification. @imalsogreg Thanks for acting so quickly.

[imalsogreg]

Latest push removes all the fields from ClientOptions except for optsRequestFixup :: XhrRequest a -> IO (XhrRequest a), as both debug-printing and withCredentials-setting can be implemented in terms of that.

I was considering setting withCredentials automatically based on BaseUrl - True for absolute paths (more likely to be CORS) and False for relative paths (never CORS)... but since the XHR javascript API doesn't do this, I figured it may be best to let library users set it always manually - they likely have a bette grasp on the right & wrong times to use withCredentials than I do :)

I'll merge this pretty soon, unless you have some other preferences?

[3noch]

@imalsogreg This is great! Thanks for your hard work on this!

[3noch]

I think you're right not to do anything fancy with withCredentials.