Feature request: strip whitespaces in BaseProcessor

[LerikP]

Proposition:
I propose to strip whitespaces in BaseProcessor and assume whitespaced values as None.

Reason:
Cells with whitespaces in large files are hard to find for users and displayed error is not clear to them.

Expected behavior:

def test_integer_processor_with_whitespace():
    processor = IntegerProcessor()

    assert processor(' ') is None

Current result:

import_me.exceptions.ColumnError:   is not an integer.