refactor: move active url check logic to a method

[beholdr]

Currently menu item marked as active even when it’s not equal, but has common prefix. Like /about and /about/something. But often it’s useful to strictly check for URL activity when both items are in the menu.

There is a discussion with solution for this case: #20 but it requires copying of ActiveUrlChecker class.

This PR moves checking logic of the ActiveUrlChecker to a separate method for easier extending of this class. It’s a simple change that will not affect current work.

[riasvdv]

Thanks!