Add CompoundPluginDescriptorFinder

[decebals]

A POC (proof of concept) for #143 (comment).

[coveralls]

Coverage decreased (-0.2%) to 54.644% when pulling af6e3c8 on compound into da346cf on master.

[coveralls]

Coverage increased (+0.3%) to 55.196% when pulling 9b2e496 on compound into da346cf on master.

[janhoy]

Looks nice. I can give more feedback once I try to move from my homegrown solution to this one. But that won't happen until next week or so.

[coveralls]

Coverage increased (+0.3%) to 55.184% when pulling 8db677b on compound into da346cf on master.

[coveralls]

Coverage increased (+0.3%) to 55.184% when pulling 95d3d1e on compound into da346cf on master.

[coveralls]

Coverage increased (+0.3%) to 55.184% when pulling 01fa88f on compound into da346cf on master.

[coveralls]

Coverage increased (+0.4%) to 55.241% when pulling 482b32c on compound into da346cf on master.

[coveralls]

Coverage increased (+0.7%) to 55.518% when pulling 1a0b585 on compound into da346cf on master.