Add value sanitization support

[jaap3]

The fields provided by this project should have some (optional) way of sanitizing their values. For example by running bleach on the field content.

e.g.:

RichTextWidget(..., sanitizer=bleach.clean) or RichTextField(..., sanitizer=bleach.clean)

This could be implemented using a custom to_python (similar to the strip option of Django's Charfield) or clean implementation (similar to the coerce option of Django's TypeChoiceField`.

Sanitation should not be enabled by default (i.e. the default could be an identify function lambda value: value). Enabling it should be strongly recommended in the documentation however.

Sanitation should also be configurable through Django settings, ideally on a per profile basis (with a fallback to a global sanitizer).

e.g.:

DJRICHTEXTFIELD_CONFIG = {
    'settings': ...,
    'profiles': {
        'basic': ....,
        'fancy': ....
    },
    'sanitizer': 'bleach.clean',  # 'dotted.path' string
    'sanitizer_profiles': {
        'basic': partial(bleach.clean, tags=['a', 'b', 'em', 'i', 'strong']),
    }
}

(dotted paths should be resolved using Django's import_string function).