New search system, including date: and pubdate: and combination

Now also accepts combination of #tag and intitle: and inurl: and author:
and the new date: and pubdate:
https://github.com/marienfressinaud/FreshRSS/issues/511
Each search prefix stop at the first space (we should add a possibility
to have quotes for multiple words)
So if you want two words in title, write "intitle:word1 intitle:word2"

Examples of dates:
date:2014
date:2014-02/2014-04 or date:201402/201404
date:P1W for the last week