ENH Allow devs to define support for third-party intervention drivers

[GuySartorelli]

Provides an extension hook for defining which file types are supported for file conversions with third-party intervention/image drivers.

Issue

• No way to use InterventionImageFileConverter with third-party image drivers #609

[GuySartorelli]

Note that there's no legitimate use case for updating the value or $supported if the driver is gd or imagick, since the logic below exactly mirrors the logic those drivers use internally.

[emteknetnz]

This reads a little non standard, seems like it should something more like

$supportedDrivers =  ['gd', 'imagick'];
$this->extend('updateSupportedByIntervention', $supportedDrivers);

if (!in_array($driver, $supportedDrivers)) {
    return false;
}

I misunderstood what this was doing

[emteknetnz]

Suggested change

	$this->extend('updateSupportedByIntervention', $format, $supported, $driver);
	$this->extend('updateSupportedByIntervention', $supported, $format, $driver);

Change around the params to make it a little more intentful what this is for

We're updated $supported so I think it needs to be the first param as in convention

I think $format is probably next important because it most cases there's probably only one third party library, so won't need to if/else it, so it's less important that $format.

[GuySartorelli]

The order of params for extension hooks isn't mentioned in our coding conventions... maybe it should be?
In this case I don't have any problem changing this order so I'll do that.

[GuySartorelli]

Done