Drop the filtering of :args and :argu from the session file.

I haven't experienced the bug that is hinted at in the comments, and I see the argument list as an essential part of the session information.
Allow to turn off the default Session menu via g:session_menu config flag.

I find it annoying that the plugin installs itself into a highly visible top-level menu, and the provided commands are quite symmetrical and easy to remember.
Merge branches 'drop-args-filtering' and 'turn-off-menu'