Active links, hash location `href`, automatic `ssrSearch`

This release brings some small improvements:

• Link can now accept a function in className for applying styles to currently active links. Read more #419
• In SSR, search string can now be extracted directly from ssrPath, e.g. pass /home?a=b to ssrPath and it will pre-fill ssrSearch #420
• Finally, after more than 2 years of waiting, links have proper href rendered when used with hash location. Before the fix, a link pointing to "/#users was rendered as <a href="/users" /> without a hash. This now can be controlled via an hrefs function defined on a router, but for built-in useHashLocation this is done automatically. See #421
• Minor improvement: use flatMap() method to flat child element in a switch. Thanks to abjfdi. See #254