Allow Reinteract to open files with default application.
Allow Reinteract to open files with default application.

This patch allows you to open non-python files from within Reinteract, for
example from the "Other Files" part of the sidebar.  Such files are opened
with the default application.  On Windows, this means using os.startfile();
on Macs, calling "open"; on Posix systems, calling "xdg-open".

rschroll/reinteract,rschroll/reinteract,rschroll/reinteract