[vipr] add image extractor

[v-delta]

Adds support for images hosted on https://vipr.im

I couldn't figure out how the tests work. If you give me a hint, I'll be glad to hand them in.

[mikf]

Thanks a lot for your contribution.

Tests for all other image host sites are the SHA1 hexdigests for download URL and metadata results. They can be generated with create_test_data.py and the a test URL as argument.

It is also possible to specify an expected URL pattern, values/patterns/types for the keyword dictionary, options, etc like here, but just a hash result is good enough for simple sites.