Expr support

[kgaughan]

expr popped into my mind today. I think it'd be a good way of solving #49 and a bunch of other issues in a very general fashion. It could be used to implement a set of filter/transform rules per feed and also more generally.

The syntax could be as follows:

[[feed]]
name = "Keith Gaughan"
feed = "https://keith.gaughan.ie/feeds/all.xml"

[[feed.filter]]
when = "<expression>"
transform = "<expression>"
action = "continue|stop"

Each field would be optional, with when evaluating to true by default, transform passing everything through by default, and action defaulting to continue. That should be general for a good portion of possible needs and could maybe be supplemented by some functions.

Named filters might also be useful. Not sure if I'll implement that, but if I were to, an extra use key could reference a named filter, with [filter.foo] being the syntax for declaring them.