Add support for "direct descendant" selectors to sel

[danielytics]

In CSS, .foo .bar means any element with class bar contained anywhere within any element with class bar, that itself can be anywhere in the document and .foo > .bar means any element with class bar that is a direct child of any element with class foo, that itself can be anywhere in the document.

That is, given:

[:div.a
  [:div
    [:div.b "X"]]
  [:div.b "Y"]]

.a .b would match [:div.b "X"] and [:div.b "Y"]
.a > .b would only match [:div.b "Y"]

Currently, sel only supports .foo .bar and not .foo > .bar. Support for direct-descendant selectors should be added.

Support can be manually obtained through the use of view, sel-* and the predicate functions, but support should be added to sel for convenience.