add histogram module (#354)

* add histogram module

Signed-off-by: tech4GT <varun.gupta1798@gmail.com>

* add option to drop gradient

Signed-off-by: Varun Gupta <varun.gupta1798@gmail.com>

* fix bug

Signed-off-by: Varun Gupta <varun.gupta1798@gmail.com>

src/Modules.js

@@ -21,6 +21,7 @@ module.exports = {
   'invert': require('image-sequencer-invert'),
   'ndvi-colormap': require('./modules/NdviColormap'),
   'colorbar': require('./modules/Colorbar'),
+  'histogram': require('./modules/Histogram'),
   'gamma-correction': require('./modules/GammaCorrection'),
   'convolution': require('./modules/Convolution'),
 }