Feature - return sequence as raw string

Add an optional argument 'as_raw' to classes Fasta and Faidx, which specifies whether to return sequences as a Sequence object or as a raw string. Default: False (i.e. return a Sequence object).
See discussions here:
https://github.com/mdshw5/pyfaidx/issues/11
Add a simple test file tests/test_feature_sequence_as_raw.py to show the difference between as_raw=False and True.