Now supports crawling more than 1 crawler. The required crawlers must be
passed in as a comma separated list of values in the --crawlers command
line option. Each crawler is processed one after another. Since the sqlite
database doesn't support concurrent writes this is meant to overcome
issues when running more than 1 instance of web2feed in parallel. Now,
only one instance should run but it goes through all the required crawlers.

Use importlib.import_module instead of __import__ to instantiate the crawlers