Release 3.1.0

[kelson42]

• [x] Secure the CI is green on git master
• [x] Update the Changelog
• [x] Update version
• [x] Create a tag on git
• [x] Secure new version is published on https://www.npmjs.com/package/openzim/libzim
• [x] Close current milestone and create new one incrementaly (a priori a minor version)
• [ ] Create new empty entry in Changelog (placeholder for future entries)
• [ ] Advertise this new version

The version of libzim on the releases page on github and on npm do not match the current version of node-libzim in master.