Improve configuration files handling

[thomasf]

• Add support for specifying the config.dirs flags multiple timesl

• Add -config.skip-dirs flag to skip non existent directories provided by config.dirs flags.

• Don't try to read config.file if it's an empty string.

• Close files opened while reading config.file and config.dirs.

• Allow .yaml and .yml as valid config suffixes inside config.dirs.

• Log module names and which files they are loaded from on start up.

[tcolgate]

Thanks!