Detect and display file format with directive override and config default

[Tanoti]

While the parser just cares about reading comment blocks, the underlying assumption is that the sls files in a fileset will be YAML. As other formats are supported, it would be good to try and detect these by reading the "#!" at the top of the file and setting a format property on the sls file object. To support this, a default could be supplied as a config argument and an override directive "format" used to tell the parser to explicitly set the property. This property can then be displayed in the doc output and maybe later used for format-specific processing.