This addresses issue #156 by allowing for multiple ignore patterns
to be specified. Rather than clobber the existing ignore option, I've
introduced a new one called ignore-patterns. That way there are as few
surprises as something involving regular expressions can have.
Multiple patterns must be separated by a comma. For example, if I
wanted to ignore all files that started with `test_` and `_`, I'd pass this
in: `--ignore-patterns="test_.*","_.*"`.
Close#156
