Log Message:
Update to 2.1

Changelog:
CHANGES FROM 2.0 to 2.1 18 October 2015

Incompatible Changes
====================

* Mouse-mode has been rewritten.  There's now no longer options for:
	- mouse-resize-pane
	- mouse-select-pane
	- mouse-select-window
	- mode-mouse

  Instead there is just one option:  'mouse' which turns on mouse support
  entirely.
* 'default-terminal' is now a session option.  Furthermore, if this is set
  to 'screen-*' then emulate what screen does.  If italics are wanted, this
  can be set to 'tmux' but this is still new and not necessarily supported
  on all platforms with older ncurses installs.
* The c0-* options for rate-limiting have been removed.  Instead, a backoff
  approach is used.

Normal Changes
==============

* New formats:
	- session_activity
	- window_linked
	- window_activity_format
	- session_alerts
	- session_last_attached
	- client_pid
	- pid
* 'copy-selection', 'append-selection', 'start-named-buffer' now understand
  an '-x' flag to prevent it exiting copying mode.
* 'select-pane' now understands '-P' to set window/pane background colours.
* 'renumber-windows' now understands windows which are unlinked.
* 'bind' now understands multiple key tables.  Allows for key-chaining.
* 'select-layout' understands '-o' to undo the last layout change.
* The environment is updated when switching sessions as well as attaching.
* 'select-pane' now understands '-M' for marking a pane.  This marked pane
  can then be used with commands which understand src-pane specifiers
  automatically.
* If a session/window target is prefixed with '=' then only an exact match
  is considered.
* 'move-window' understands '-a'.
* 'update-environment' understands '-E' when attach-session is used on an
  already attached client.
* 'show-environment' understands '-s' to output Bourne-compatible commands.
* New option: 'history-file' to save/restore command prompt history.
* Copy mode is exited if the history is cleared whilst in copy-mode.
* 'copy-mode' learned '-e' to exit copy-mode when scrolling to end.