feat(custom-commands): Expand custom command arg with placeholder and…

[szpanel]

Please verify the following:

• [x] yarn build-and-test:local passes
• [ ] I have added tests for any new features, if relevant
• [x] README.md (or relevant documentation) has been updated with your changes

Describe your PR

Recently I dealt with RN upgrade in our project and I started looking around for dev tool alternatives and I found Reactotron. After I configured it seamlessly in our project, I started playing around with custom commands and thought it would be cool to have an input whose data isn't visible. That's why I decided to contribute.

I'm moderately satisfied with the example I added to the ReactotronConfig because it's not perfectly intuitive, but I'm counting on your opinions.

I noticed comments in custom-commands so I refactored a little.