No way to use InterventionImageFileConverter with third-party image drivers

[GuySartorelli]

libvips is reportedly a very efficient alternative for GD or Imagick, and has at least one implementation of a third-party driver for intervention/image in osiemsiedem/intervention-image-vips-driver.

There may be other third-party drivers for intervention/image as well.

Currently if the driver isn't either gd or imagick, InterventionImageFileConverter assumes it can't support any file conversions.
An extension hook should be provided so that developers opting to use a third-party drivers can define which file types are supported.

PRs

• ENH Allow devs to define support for third-party intervention drivers #610