Feature Request: Add support for custom visitor keys.

[ota-meshi]

I want esquery to add support for custom visitor keys.
Currently, some queries crash when using a node type unknown to estraverse.
e.g. ":nth-child(1)" (See also to #110, #67)

I think that the node type decided by estree will be fixed by version upgrade, but using JSX and TypeScript nodes will continue to crash.

If esquery can support custom visitor keys, passing custom visitor keys for JSX and TypeScript allows people to use esquery in JSX and TypeScript nodes without crashing.

[octogonz]

Another option would be to simply fail to match unrecognized nodes.

Where esquery does this:

esquery/esquery.js

Lines 309 to 312 in e27e73d

	const keys = estraverse.VisitorKeys[parent.type];
	for (const key of keys) {
	const listProp = parent[key];
	if (Array.isArray(listProp)) {

...it could avoid crashing by simply doing this:

  const keys = estraverse.VisitorKeys[parent.type]; 

  if (keys === undefined) {
    return false;  // <--------------------
  }

  for (const key of keys) { 
    const listProp = parent[key]; 
    if (Array.isArray(listProp)) { 

[octogonz]

Also, fixing #114 would probably cause the query engine to skip a lot of cases where this is encountered.