Expose a public function to access active state (#49)

A simple use case could be setting `last_status = 3` when the statusline
is not embedded in tmux.

Author: guruor

autoload/tpipeline/state.vim

@@ -28,6 +28,10 @@ func tpipeline#state#toggle_frozen()
 	endif
 endfunc
 
+func tpipeline#state#is_active()
+	return s:active
+endfunc
+
 func tpipeline#state#restore()
 	let s:active = 0