Search syntax: '+' should mean 'any category' about elfeed

[skeeto]

(In my response, for consistency I'll be using the standard Elfeed terms
for these things: s/articles/entries/, s/categories/tags/.)

I like this train of thought. Right now a lone + will find all entries
tagged with the empty symbol ('##). Perhaps someone out there is taking
advantage of this, but I think that's a pretty useless tag, so I'm fine
with taking away the ability to use it.

As a side note: for quickly switching between different common filters,
some people bind keys (elfeed-search-mode-map) to change the filter in
effect without needing to manually edit the filter. This would allow you
to use full tag names, rather than single characters, without having to
type them out.