Add default true persist option to display positions with persistence

Setting --persist=0 will cause only the current detected contact positions
to be plotted.  This behavior can be toggled at run time by pressing the
'p' key.

BUG=chromium-os:29063
TEST=mtplot --persist=1 # shows persistent dots
TEST=mtplot --persist=0 # does not show persistent dots

Change-Id: I1d70dd89e8880423a26fae4534c01ec7a0658d1e
1 file changed