1. Allow a directory path, default to ".", and read all the WiredTigerStat* files in the directory

2. WiredTiger generates a lot of statistics, and by default, wtstats.py creates one graph including all of the statistics, which is hard to manage for humans and browsers. Investigate whether newer versions of NVD3 offer anything to help, and if not, add a simple way to generate multiple graphs with fewer statistics in each one.