Bugfix: Support thresholds for bar charts

[Hamsterbau]

Like mentioned in the documentation, threshold plugin should also work for bar charts. But currently plugins are only usable for line charts because of missing support for plugins in bar chart.

LineRenderer.java:

if (chart.getPlugins() != null) {
	writer.write(",plugins:" + chart.getPlugins());
}
```		

**BarRenderer.java:**

No part for handling plugins.

Fix should be easy, just add the lines from LineRenderer.java to BarRenderer.java.