This should work on linux (xdg-open) and OSX (open). If the program
is not in $PATH, it falls back to opening a browser window/tab with file:///
The only tricky bit is the javascript code, that handles clicking on the
link. This is to avoid unnecessary page refreshes. Until I added the
return false at the end, the <a>'s normal click event also fired, so two
file browsers opened. I have not checked portability extensively.
