Add config_file attribute

Search for `jsl.conf` in the linted file's working directory, its parents, and the home directory.
Remove defaults attribute

defaults['-conf:'] no longer needed with config_file attribute
Update explanation of jsl config files in README

Explain how to specify a custom config file in the "args" option, and
indicate that the plugin will automatically search for a config file if
none is specified.
Merge pull request #3 from jawshooah/add-config-file

Add config_file attribute