Glide user defined default support

[SanderFlooris]

trafficstars

I ran into the situation where I wanted to add compression by default, but it seems like there is currently no way to do this.

Inside src/Imaging/ImageGenerator.php where the defaults are loaded, you can see that a todo was placed here, but that has been there for quite a while.

Location of the TODO: https://github.com/statamic/cms/blob/e7f48d1e1063c82651b6e9ebe4feef8d3770df23/src/Imaging/ImageGenerator.php#L288