Move omit_if_none and omit_if_empty properties to unit class.
Extend unit init attributes.
Use `get_metadata` on validator instance to retrieve metadata.
Unit is required by default.
Set parent to unit.
Lookup appropriate function by `isinstance`.
Add new validators.

- ValidatedChain to validate several validators as one.
- Regex to validate regular expressions.
- Email to validate email.
FileFieldStorage unit type.
Add context property.