feat: custom cli flags

[shellscape]

This PR contains:

• [ ] bugfix
• [x] feature
• [ ] refactor
• [x] tests
• [x] documentation
• [ ] metadata

Breaking Changes?

• [ ] yes
• [x] no

Please Describe Your Changes

This PR introduces easy access to custom flags specified on the command line. This module doesn't validate CLI flags and as such allows the user to use any number of flags, defined in any way desirable. This PR adds a new export for easy access to custom flags and negates the need for inline envars.