Refactor parsers submodule

[atteggiani]

Refactor parsers submodule to simplify the creation of all the different parsers.

• 4 parsers exist: MainParser, SubCommandParser, help Parser, common Parser. Are they all needed?
• Fix triple quoting indentation. Fix formatting of multiline triple quotes with \n chars. Confirm automatic dedentation in triple quoted strings.
• Replace classes with function calls? (simplify the code)
• Merge modules into single module?
• Cleanup formatting parentheses in callback_function()
• Simplify the logic?