Enh: add maintenance script to rename a key in locales

[Axolotle]

Added a script so that you can rename a nested i18n key in 'en' or all locales.
This won't change the value, only move the key.
Also added commands to README.md

Examples:

If you need to rename a key or more (from 'my.current.key' to 'my.new.key' for example).

From a string

python3 rename_i18n_keys.py --keys my.current.key:my.new.key

From a file

python3 rename_i18n_keys.py --file input.txt

input.txt

my.current.key:my.new.key
my.other.key:my.new.other.key

By default it renames keys only in the en.json, pass --all to apply changes to all locales file.