Changing -c option to allow specifying the full filename to the config file

[tilo]

there are so many config files around, that I think it's better if the config-file is fully specified with the -c option, rathrer than having to guess that the filename is 'config.yml'

Also checking that the filename exists and it's a file.

[coveralls]

Coverage remained the same when pulling 66a2a02 on tilo:feature/yaml-change-config-option into 8753fe3 on 9to5:feature/yaml.

[coveralls]

Coverage decreased (-0.29%) when pulling 9d2e975 on tilo:feature/yaml-change-config-option into 8753fe3 on 9to5:feature/yaml.

[tilo]

Closing this - there's a better way to do this.