findimports should ignore hidden directories by default

[mgedmin]

E.g. when I run findimports in its own source directory, it tries to parse .tox/py27/lib/python2.7/UserDict.py and fails with a SyntaxError. (I'm running findimports with Python 3.7).

[baxelrod-bdai]

Maybe also ignore files listed in .gitignore and .ignore similar to other code searching tools like ripgrep, ag, etc.