added a way to inject additional article hints into ReadSharp

[alexeib]

NReadability has some hard coded settings for being able to extract content from specific sites where automatic extraction fails. They do this by having a dictionary of site url -> html element selector containing the content of the article.

Unfortunately they do not list all possible sites that have this problem and there is no way to inject additional sites. This changeset allows one to optionally pass the same url -> selector dictionary down to nreadability to be added to their hardcoded list. Also adds slashdot to the hardcoded list while we're at it :)