Add NVD3's bullet chart (#1775)

* Add NVD3's bullet chart.

* Add empty lines before nested function definitions.

* Add thumbnail for bullet chart.

* Add bullet chart to gallery.rst.

* Add "requiresTime: false", fix indentation.

* Avoid scaling bullet chart vertically.

* Use a default if no range is specified.

* Fix coloring of bullet chart.

superset/assets/visualizations/main.js

@@ -6,6 +6,7 @@ const vizMap = {
   big_number_total: require('./big_number.js'),
   box_plot: require('./nvd3_vis.js'),
   bubble: require('./nvd3_vis.js'),
+  bullet: require('./nvd3_vis.js'),
   cal_heatmap: require('./cal_heatmap.js'),
   compare: require('./nvd3_vis.js'),
   directed_force: require('./directed_force.js'),