feat: add global default params

[Duder-onomy]

I wanted to be able to specify some default params from the applications config. Specifically, I wanted the auto='format' param. While implementing, I noticed the other global config options were untested so I added some tests for those while I was in there.

Description

• Adds tests for current global configuration options [debug, classNames, disableLibraryParam]
• Adds tests and feature for NEW global configuration option called defaultParams which is an object of default params that will be applied to all src's.
• Adds notes to the readme.

New Feature

• [NA] If this is a big feature with breaking changes, consider [opening an issue][issues] to discuss first. This is completely up to you, but please keep in mind that your PR might not be accepted.
• [✔️] Run unit tests to ensure all existing tests are still passing
• [✔️] Add new passing unit tests to cover the code introduced by your PR
• [✔️] Update the readme
• [✔️] Add some steps so we can test your cool new feature!
• [✔️] The PR title is in the conventional commit format.

Steps to Test

• Set a imgix param via the applications config, something like:

///
APP: {
    imgix: {
        path: ...,
        defaultParams: {
            auto: 'format',
        },
    },
},
///

• Observe that param being applied to all src's.
• Pass a param via the options hash or as params on the url.
• Observe that all other param setting methods take precedence over the global values.

[frederickfogerty]

Hey @Duder-onomy I'm currently taking a hiatus from imgix as I complete my university degree - I'm aiming to be back on board sometime later this year. Until that point, Jason (@jayeb) from imgix is the man you should reach out to for PR reviews or other questions!

Anywho, I've given this a look over and it seems fine to me. @jayeb or @sherwinski should probably give this a review as well before it's merged, as they're more in-sync with imgix's internal plans.

[frederickfogerty]

LGTM 👍

[jayeb]

This looks good, but I do have a question about the fallback for setting classes.

[jayeb]

Would there ever be a case where someone would want no default class on the img element? This || 'imgix-image' would make it impossible to unset this option entirely.