Feature Request: Make svg dependencies optional

[MarcoGlauser]

Hi, thank you for maintaining django-filer for such a long period and keeping it up to date!
svglib, which is used by easy_thumbnails[svg], which is a dependency of django-filer has changed how some dependencies are handled. This broke our CI. Pycairo is now required and doesn't provide wheels, which requires quite a few additional os libraries to be able to install.
See this issue from svglib:
deeplook/svglib#413

We don't use any svg files with filer and want to avoid unnecessary bloat to our images where possible.
Our current workaround is to pin svglib to an older version, that didn't require additional os libraries to be installed.

Would it be possible to make easy_thumbnails[svg] as an extra and default to the default easy_thumbnails?

[fsbraun]

@MarcoGlauser For the time being, I suggest to pin svglib to svglib!=1.6.0 in your project.