Make regexp strings raw
Make regexp strings raw

Fixes Python 3.6 warnings:

    waybackpack/asset.py:20: DeprecationWarning: invalid escape sequence \d
    waybackpack/asset.py:21: DeprecationWarning: invalid escape sequence \s
    waybackpack/asset.py:22: DeprecationWarning: invalid escape sequence \?

jsvine/waybackpack