add clojure.java.browse/browse-url

[borkdude]

As discussed on Slack:

@lread I would be open to a PR that adds clojure.java.browse/browse-url, but only shells out and doesn't use the reflection or swing stuff. (edited)
So our own stripped down implementation of it.

[lread]

A note from lukasz on Slack:

@lread FWIW - open and xdg-open do all of this for you in OSX and Linux respectively, so could be just a one-liner. No idea about Windows

[borkdude]

The Windows equivalent of open/xdg-open is probably start on Windows. The clojure.java.browse namespace has also some logic for Cygwin, but I'm not sure how relevant this is in 2020.