Make sure vim-misc is installed, politely complain if it isn't

See issue #65 on GitHub and numerous other issues before it:
  https://github.com/xolox/vim-session/issues/65
Add session_default_overwrite option (see readme for details)

See also issue #62 on GitHub:
  https://github.com/xolox/vim-session/issues/62
Support for sessions with multiple TagList windows

See issue #66 on GitHub:
  https://github.com/xolox/vim-session/issues/66