Varvault

Made a fix for the parameter `default` when using it with `get`. It had no effect, even when the flag `input_key_can_be_missing` was configured. Also added a test for this to make sure it works as expected.

Removed a rogue print message in `vaulter` function that got left behind in the previous release

Significant changes:
* Made some changes to how filehandlers are handled to make them more customizable to theoretically allow a user to run varvault towards a database.
* This has caused some APIs to have to change since they didn't make any sense anymore, hence the new major version.
* Moved validators to a separate new file validators.py since it didn't make much sense to have it in keyrings.py.
* Scrapped the VaultInterface class. It makes no sense and we don't need it for a user to overload the class, which was its original intent.
* Removed some functions under utils.py as they don't make sense to have there anymore.
* Scrapped the constants in class VaultFlags (vaultflags.py) and instead we just use the name of the function as the constant that names the actual flag.
* Made it so 'build_from_vault_key' in class VaultStructBase (vaultstructs.py) raises NotImplementedError if the function isn't implemented. Python handles abstract classes weirdly when the abstract class also inherits a bultin type (dict, list, float, etc) and allows you to create an instance of a class that doesn't implement all abstract methods, which causes some weird bugs.
* The JsonFileHandler class has been moved to __init__.py instead of filehandlers.py.

Added a function called `add` to `MiniVault` which essentially wraps the `__setitem__` function. This change aims to make it a little bit more easy to add variables to a MiniVault-object.

Made a fix for a bug where if a key was defined without a valid type (i.e. valid_type was set to None), the key could not be parsed from an existing vault file. Also added a test for it.

Added functionality for passing one or more validation functions to a key in a keyring to validate a key in much more detail than just checking that the type is correct.

For example, it's possible to set up a key whose valid type is int and pass a validation function to the key to validate that the value is an even number. Alternatively, you could validate the length of a list to a certain number of elements, or make sure that a dict-object contains a specific set of keys.