Feature to export JSON backup file to reimport later, with all stars and tags

[gabrielgodoy-zz]

Add feature to export a JSON file that can be reimported later, on some other account for example.

When imported to a new account, this JSON file contain all previous starred repos, and the extension should star them if not starred, and insert all the preexistent tags.

This way, if you have a JSON backup file, you not loose the tags if you remove the extension for some reason. You can reinsert via this JSON file.