Make fish bindings feature complete, add documentation

Added features:
- Updating TMUX_PWD_* when PWD variable changes
- Updating TMUX_COLUMNS_* when WINCH signal is received

docs/source/overview.rst

@@ -150,6 +150,16 @@ absolute path to your Powerline installation directory:
 
    . {repository_root}/powerline/bindings/zsh/powerline.zsh
 
+Fish prompt
+^^^^^^^^^^^
+
+Add the following line to your :file:`config.fish`, where ``{repository_root}`` 
+is the absolute path to your Powerline installation directory:
+
+.. code-block:: bash
+
+   . {repository_root}/powerline/bindings/fish/powerline.fish
+
 Tmux statusline
 ---------------
 

powerline/bindings/fish/powerline.fish

@@ -6,7 +6,7 @@ function powerline
 			set -g -x POWERLINE_COMMAND powerline
 		end
 	end
-	function _powerline_update -v POWERLINE_COMMAND
+	function --on-variable POWERLINE_COMMAND _powerline_update
 		eval "
 		function fish_prompt
 			$POWERLINE_COMMAND shell left --last_exit_code=\$status --last_pipe_status=\$status
@@ -17,4 +17,19 @@ function powerline
 		"
 	end
 	_powerline_update
+	function _powerline_tmux_setenv
+		if test -n "$TMUX"
+			tmux setenv -g TMUX_$argv[1]_(tmux display -p "#D" | tr -d "%") "$argv[2]"
+			tmux refresh -S
+		end
+	end
+	function --on-variable PWD _powerline_tmux_set_pwd
+		_powerline_tmux_setenv PWD "$PWD"
+	end
+	function --on-signal WINCH _powerline_tmux_set_columns
+		_powerline_tmux_setenv COLUMNS "$COLUMNS"
+	end
+	_powerline_tmux_set_columns
+	_powerline_tmux_set_pwd
 end
+powerline