Add `store_file` option

Added `store_file` option to store internal data into a file of the given path on shutdown, and load on starting. 

With this option, I could avoid a graph (typically, of GrowthForecast) to drop out when I restart fluentd. 

I also added `Paramters` section on README, and changed the README format from rdoc to markdown because it seems moris-ware usually use markdown for README.