Feature: Make --graph-output less surprising and more usable

[jhermann]

If isatty(sys.stdout), then do not spew a PNG or similar not-so-legible stuff onto the console (I did have a WTF moment), instead write it to a tempfile and either webbrowser.open_new_tab() it, or call xdg-open with it (if that's on the PATH).

[gaborbernat]

I like what this wants to achieve but needs more research how this can work cross platform.