Revise filter API

[jgm]

Perhaps it should work as in pandoc:

• If you don't return a value, the node doesn't change.
• If you do, the node is replaced by the node you return.
• If you return an array of nodes, they are spliced in at the position of the node. (This can be used to delete elements by returning an empty array.)

[jgm]

Commit 8380cd8fb7dcd22cc0dcfac9396dd87774c742aa contains some steps towards this. The main problem I see is in figuring out a new way to handle "stop traversal," rather than returning true.

Here are some comments on the new API, to be inserted in filter.ts eventually:

 *
 * The filters we've looked at so far modify nodes in place by
 * changing one of their properties (`text`).
 * Sometimes we'll want to replace a node with a different kind of
 * node, or with several nodes, or to delete a node.  In these
 * cases we can end the filter function with a `return`.
 * If a single AST node is returned, it will replace the element
 * the filter is processing.  If an array of AST nodes is returned,
 * they will be spliced in to replace the element.  If an empty
 * array is returned, the element will be deleted.
 *
 * // This filter replaces certain Symb nodes with
 * // formatted text.
 * const substitutions = {
 *   mycorp: [ { tag: "str", text: "My Corp" },
 *             { tag: "superscript",
 *               [ { tag: "str", text: "(TM)" } ] } ],
 *   myloc: { tag: "str", text: "Coyote, NM" }
 *   };
 * return {
 *   symb: (e) => {
 *     const found = substitutions[e.alias];
 *     if (found) {
 *       return found;
 *     }
 *   }
 * }
 *
 * // This filter replaces all Image nodes with their descriptions.
 * return {
 *   image: (e) => {
 *     return e.children;
 *   }
 * }