pingplot: Add -S: Auto-split data into single days

$ pingplot -r 2014-02-19-16-08-26.log -S -s 4096x700
Image for 2014-02-19: /tmp/tmp.10.1.10.1-2014-02-19.UhGx0muvRt.png
Image for 2014-02-20: /tmp/tmp.10.1.10.1-2014-02-20.GZN5pE5CWa.png
Image for 2014-02-21: /tmp/tmp.10.1.10.1-2014-02-21.7ghWFtXrQY.png