Apply rules more selectively; add selection options

[itohsnap]

Fixes https://github.com/dewey/miniflux-sidekick/issues/13 - all rules in the killfile were being applied to all feeds that had any rule specified, regardless if a given rule applied to a specific feed. (This also attempts to validate filter expressions when loading rules, instead of when trying to apply them.)

Adds the ability to set Filter Expressions involving post authors, and to use Miniflux categories instead of a specific feed URL, in killfiles.

(I'm still not a Go programmer... but this all seems to work for me.)