Argparse and Object-oriented refactoring

Just finished `argparse` implementation. I just made another refactoring : created a class for Sheets, just one instance for all with dirs and file at `__init__`.

`Argparse` works nice, just created 3 custom `argparse` actions to call `sheet.edit`, `sheet.list` and `sheet.dirs`. The print is made in the main, easier to add something like subcommands or so.

If OO refactoring isn't fine, my master branch contains the same things withouth `CheatSheets` class.

Hope it helps. I am here to awnser questions if needed.