Define menu items for commands defined by plug-in (issue #41)

Issue #41 on GitHub:
  https://github.com/xolox/vim-session/issues/41
Support for Conque Shell (issue #17)

Issue #17 on GitHub:
  https://github.com/xolox/vim-session/issues/17
Make xolox#misc#os#exec() report exit codes (breaks backwards compatibility)

Working towards fixing this issue: xolox/vim-lua-ftplugin#8
Updated miscellaneous scripts
Bug fix for automatic, periodic session saving (issue #26)

Issue #26 on GitHub:
  https://github.com/xolox/vim-session/issues/26
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.