Changes

• Add Heatmap class (and HeatmapColor) to support the Heatmap panel (#170)
• Add BarGuage for creating bar guages panels in grafana 6
• Add GuagePanel for creating guages in grafana 6
• Removed gfdatasource - feature is built in to Grafana since v5.
• Generate API docs for readthedocs.org
• Fix AlertList panel generation
• Add both upper and lower case "time" pattern for time_series column format in Table class
• Drop testing of Python 2.7, it has been EOL'ed and CI was broken due to this.
• Automatically test documentation examples.
• Point to dev meeting resources.
• Add description attribute to Dashboard.
• Add support for custom variables.
• Point out documentation on readthedocs more clearly.
• Add average metric aggregation for elastic search
• Bugfix to query ordering in Elasticsearch TermsGroupBy
• Added all parameters for StringColumnStyle
• Add Elasticsearch Sum metric aggregator
• Add Statusmap class (and StatusmapColor) to support the Statusmap panel plugin
• Bugfix to update default Threshold values for GaugePanel and BarGauge
• Use Github Actions for CI.
• Fix test warnings.
• Update BarGauge and GaugePanel default Threshold values.
• Update release instructions.

Thanks a lot to the contributions from @DWalker487, @bboreham, @butlerx, @dholbach, @franzs, @jaychitalia95, @matthewmrichter and @number492 for this release!