support email format validator

Before:

```python
>>> import validictory
>>> validictory.validate("foo", {"type":"string", "format":"email"})
>>> 
```

After:

```python
>>> import validictory
>>> validictory.validate("foo", {"type":"string", "format":"email"})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "validictory/__init__.py", line 31, in validate
    return v.validate(data, schema)
  File "validictory/validator.py", line 574, in validate
    self._validate(data, schema)
  File "validictory/validator.py", line 577, in _validate
    self.__validate("_data", {"_data": data}, schema)
  File "validictory/validator.py", line 608, in __validate
    newschema.get(schemaprop))
  File "validictory/validator.py", line 470, in validate_format
    format_validator(self, fieldname, value, format_option)
  File "validictory/validator.py", line 91, in validate_format_email
    "not an email" % locals(), fieldname, value)
validictory.validator.FieldValidationError: Value 'foo' of field '_data' is not an email
>>> 
```