Default negative values support confuse people

[michael-yin]

version 0.10.x Supports negative values by default, if we enable countdown with flipclock, then we would see -0:00 in the end, but I think 00:00 would be better here (like the old version).

The - sign in the face sometimes confuse people. I think we better add option to let developer disable this feature.

[ericchan3721]

I believed this issue is related to the getTotalSeconds() in flipclock.js

After calculated the time difference, the Math.round() just rounded the diff value likes -0.0... as -0, not 0

Therefore, the negative sign shows when countdown to zero