Add option to pass custom data

[plm75]

This adds the option to pass custom data to each parsed article through the meta property.

[danmactough]

@plm75 Can you tell me why this is useful? Why do you need to pass this through on meta instead of just referring to it directly?

[plm75]

@danmactough I need this for when I fetch several feeds concurrently and need to pass specific data for each article.