Add the possibility to unlock a project

[2ndkauboy]

Issue Overview
I'm just started using this plugin, and it's amazing! But just as @ocean90 mentioned in #218, I ran into an issue and couldn't find out what it was. If you are sure you resolved an issue and want to restart, the update process would not run as the project is locked. I have not seen any code that would allow me to unlock it. I'd suggest that either with wp traduttore project update and the --delete flag set, it should unlock it or when you run wp traduttore project cache clear for a project.

To Reproduce
Steps to reproduce the behavior:

1. Run an update of a project that fails (e.g. critical PHP error)
2. Run the update again
3. The "Warning: Could not update translations for project (ID: 1)!" will be shown

Expected behavior
Some CLI commands should also remove the lock.