Ticks support

[hecko]

Would love to see the ticks support in the gauges.

For my user case I would love to see a possibility to define OK, WARN, CRITICAL levels of a gauge value which would be visible by defining the ticks (or thresholds) in the gauge. Currently we use colors to do this, however colors are sometimes difficult to interpret by our users especially when they are not used to gauge to move too much and don't see the change in color in typical operation.

This might also improve accessibility of using the plugin in atypical operations.