Installing Addon
You can either unzip and copy weights_extra folder to your blenders addons folder or you go to blender preferences -> add-ons tab, click install and select weights_extra.zip file. Make sure Weights Extra addon is activated by searching its name on addons panel.

Tool Panel
When you activate Weight Paint mode, tool will show up at the bottom of Tool panel.

Exporting Weights
Click export button. It only exports vertex weights used by armature, all other vertex weights will be ignored.

Importing Weights
Click import button and select the import method if needed. Its default to 'Auto' and will pick best method that fits to current import scenario. It replaces only armature vertex groups, all other vertex groups stay intact.

API Methods
You can use its python api to import your own scrips.

from weights_extra import import_weights, export_weights

import_weights(object_name, filepath)

export_weights(object_name, filepath, method="AUTO") 
# method is optional, default is "AUTO", other options are "VERTEX_ID", "CLOSEST_VERTEX", "CLOSEST_POINT"

Please report any bug or request by contacting support at instantrig.com

Hope you find this tool useful, enjoy.