RSS/Atom: add a parameter to add tags to the flux

[tolima]

if you use shaarli with seenthis or twitter the tags are in the text with the key character #. And you have to put them twice. I think the tag field is useful, if you don't want your tags in the description but most of the time it help to know quickly what is about.

ps : may be with an additional parameter to filter tag you don't want in the flux (ex: tag twitter to tweet the link but we don't want the "tweeter" tag in all our tweet)