Send UTM params

[aaronklaassen]

@lukechesser @HughbertD

These days the API terms have this requirement: https://community.unsplash.com/developersblog/unsplash-api-terms-explained#block-yui_3_17_2_1_1490972762425_202608

But without being able to send arbitrary query params via the lib, clients can't do that. Since it should be attached to every request anyway, maybe the best way to do this is to make the utm_source value part of the lib config, and everything just gets attached automatically. i.e.,

Crew\Unsplash\HttpClient::init([
	'applicationId'	=> 'YOUR APPLICATION ID',
        'secret'        => 'YOUR APPLICATION SECRET',
	'callbackUrl'	=> 'https://your-application.com/oauth/callback',
        'utmSource'     => 'my sweet app'
]);

or whatever.

For backwards compatibility, I'd log a warning if that value is missing from the config, but not fail outright.

[HughbertD]

@aaronklaassen If the user hasn't given their utmSource, do you want the links to either fall to a default and generate something like:
?utm_source=nottracked&utm_medium=referral&utm_campaign=api-credit

Or skip adding UTM parameters entirely? Wondering if some data is better than none.

[aaronklaassen]

@HughbertD Good question. I'll get back to you.

[aaronklaassen]

@HughbertD Yeah put a placeholder value in - api_app works.

[lukechesser]

@aaronklaassen good thinking on this 👍