Readability support

[Necoro]

As stated in #66 (comment):

It scrapes a website for value content only and removes all the clutter. This can be used in some instances to turn a feed with preview content only into a full feed.

Instead of using the proposed https://github.com/go-shiori/go-readability, I created a trimmed down fork that avoids too much clutter in the dependencies.

[Necoro]

@snipem: Turns out, I unexpectedly had some time at my hands. So the readability support is included in general. I need to run some more tests to see if performance is satisfactory.

You can enable that in the feed options by setting the body option to a new-gained value fetch

[Necoro]

Implemented with 1.5.0