Invoke :Browse if available to open URL

Example that invokes open(1) on OS X:

    command! -bar -nargs=1 Browse silent! !open <args>

Closes #509.

plugin/fugitive.vim

@@ -2147,6 +2147,8 @@ function! s:Browse(bang,line1,count,...) abort
     if a:bang
       let @* = url
       return 'echomsg '.string(url)
+    elseif exists(':Browse') == 2
+      return 'echomsg '.string(url).'|Browse '.string(url)
     else
       return 'echomsg '.string(url).'|call netrw#NetrwBrowseX('.string(url).', 0)'
     endif