Issue #5 - Save volume of the mpv player

[Hibbins]

Closes #5

📑 Description

I added the following:

• Default configuration file, that can keep options, such as volume etc.
• Lua script that gets the volume from mpv and saves it to the configuration file
• Made sure the Lua script is being run together with mpv, using the --script flag
• Added function that gets the volume value from the configuration file whenever the play function is called

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

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[Hibbins]

Please let me know what you think of the proposed solution! 🙂 @shinokada