Add option to limit search result by distance

If you have many stores, e.g. all over US and user searches for New York,
allow the website to show only stores within a certain distance, e.g. in 50
miles radius of New York.

Feature is turned off by default (the current stores setting) and can be turn
on by setting STORES_MAX_SEARCH_DISTANCE for central limit or can be modified
by overriding get_max_distance() for dynamic values, e.g. changing the
distance for certain queries.