Hide empty collectors until used

[barryvdh]

This will hide empty collectors, until the collector is actually filled using any data.
This can be useful for less-used collectors without filling up te screen.

Note:

• It is not hidden when empty, to keep a more consistent flow when using ajax etc.
• it's not using the badge because not every collector uses a badge

Not sure if it's worth creating a config option for.

[barryvdh]

Or should we have a threshold for the collectors, so we can make it configurable?

[barryvdh]

Added a config flag