Decoders: better error handling on malformed input

[boutros]

Right now the decoders all give up parsing when there is some malformed RDF input.

I'd like to also make it possible to ignore errors and try to continue parsing, discarding only the malformed parts. This is easy in line-based formats N-Triples & N-Quads, but more difficult in Turtle & RDF/XML.

Start by adding an ParserOption ("Strict"?), which should indicate the desired behaviour. Default to true, but if set to false, it will try to continue, skipping the malformed input.

Maybe also add an option (an io.Writer perhaps, defaults to nil) so consumers can decide where to print errors & warnings.