Add start-or-switch-to defun.

Seems handy, not using it yet. Borrowed from:
http://emacsredux.com/blog/2013/04/29/start-command-or-switch-to-its-buffer/
Auto-set mode based on buffer name.

New buffers created via switch-buffer are always created in the
default major-mode. Added a hook to set the mode using the
auto-mode-alist based on the buffer name for non-file buffers.

An example:

  C-x b foo.org RET
-> create buffer foo.org and set to org-mode

 M-x rename-buffer foo.md
-> rename buffer and set mode to markdown-mode