add additional custom_filter examples

[AndyCarnevale]

Added two examples that further demonstrate how to use the custom_filter parameter. These examples should make it easier to understand how to filter out specific feature keys and options.

Examples expand upon the existing New York subway example, and reuses code from this Stack Overflow answer: https://stackoverflow.com/a/66320085

Also added a link to the relevant Overpass API documentation for additional help.

[gboeing]

@AndyCarnevale tests are failing.