Allow filter attributes in wpCustom

[emhoracek]

The idea was that you could use filters like limit=5 and offset=20 in <wpCustom> tags as well as wpPosts.

I started working on this here: #40

But I then I realized that 1) many of the "custom" endpoints you're going to be using here don't have all these query options and 2) this needs to work even if a user puts query params in the endpoint (example: <wpCustom endpoint="authors?search=something" limit=5> shouldn't result in a query of authors?search=something?limit=5).

Since technically you can already do all these queries with query params directly, I decided not to implement the feature at this time, but I still think it would be nice for consistency's sake.