Added multimedia keyboard controls

[LexiconCode]

Description

Edit commands to control keyboard multimedia keys.

Related Issue

None

Motivation and Context

This allows people to manipulate there multimedia playback without an application being in focus.

How Has This Been Tested

Tested with YouTube in Firefox

Types of changes

• [ ] Docs change / refactoring / dependency upgrade
• [ ] Bug fix (non-breaking change which fixes an issue or bug)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Renamed existing command phrases (we discourage this without a strong rationale).

Checklist

• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [x] I have checked that my code does not duplicate functionality elsewhere in Caster.
• [x] I have checked for and utilized existing command phrases from within Caster (delete if not applicable).
• [x] My code implements all the features I wish to merge in this pull request.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests pass.

Maintainer/Reviewer Checklist

• [x] Basic functionality has been tested and works as claimed.
• [x] New documentation is clear and complete.
• [x] Code is clear and readable.