Add `parseBuffer` method

[paya-cz]

My RSS feed has encoding="ISO-8859-1" in the header. I cannot use parser.parseURL because I need to implement custom error handling and rate limiting. The only alternative is using parser.parseString, but to use it I would need to already have a decoded string. I only have a Buffer with unknown encoding, and I would like to pass it to rss-parser to decode. I think a new function parser.parseBuffer would be appropriate for this task, and I imagine it can share a lot of code with parser.parseURL.

[rbren]

👍 this seems like a great idea!