Custom max name length

[pimuzzo]

This feature is essential to be able to use Djongo on AWS with DocumentDB.

Please note:

• that according to this StackOverflow comment the AWS declared limit is 63 chars but it supports fewer chars.
• that this edit is backward compatible.

It can be used in this way:

DATABASES = {
    'default': {
        'ENGINE': 'djongo',
        [...]
        'MAX_NAME_LENGTH': XX,
    }
}

Fix https://github.com/doableware/djongo/issues/595