Feature/cleaning up aggregate indexer fallback (#1507)

* Line endings...

* Refactoring how MetaIndexers handle fallbacks

Originally this modification was part of a much larger refactoring,
however for the sake of reviewability I split it into smaller chunks.
Sadly it is still quite large.
I wanted to split it even more, however after a certain point there was
really no value in creating smaller chunks. The biggest part of this
modification would be still huge.

So all in all, there're 3 aspects of this modification
- It modifies BaseIndexer so that it now implements IIndexer (will be
very useful later on)
- Resolving most of the warnings currently in Jackett (the only ones
remaining are related to Autofac, however if I could I would just burn
Autofac altogether rather than fix the warnings. Will open discussion on
this.)
- Biggest part: refactoring how MetaIndexers handle fallbacks and how
they provide the final result set

MetaIndexers now accept any kind of fallback and filtering mechanism
that implements the necessary interface, so that in the future IMDB
fallback and filtering won't be the only one. I know there are not a lot
of unit tests around Jackett at the moment, however this renders the
class much more unittestable as well.

* Autofac started complaining... I don't understand...