Support multi-words separated by a whitespace

[ArthurHoaro]

There is no rule on how tags should be separated (and for anything for that matter), and we have a few unit tests where tags are actually separated by a whitespace.

However, it seems that the comma is a common separator for tags.
Regarding shaarli/Shaarli#594, I suggest that if tags contains a comma, we use the comma as a separator, otherwise we use the whitespace.