Add a `before` and `after` filter

[esoterra]

Add a filter that removes all items with a .date field less than the provided date and a filter that does the opposite.

e.g.

sections.pages | before("2020-03-10")

sections.pages | after("2020-03-10")

Use Case

I want to render talks I gave before a certain date in a different list than ones after.
I'm currently doing this by adding an extra frontmatter key into my pages but I'd rather do this by date and this would make that much easier.

Naming

I think before and after would be good names for this.

However, if these collide with existing names for anything, some prefix could be added to make them more unique like dates_before and dates_after.