Update opml.php to export TTL

So far, tt-rss does not export all the fields of the ttrss_feeds table. As a starter, I have some interest for the update_interval parameter.
So here is a patch for adding TTL (for update interval in minutes) for each outline, if specified (i.e. different from 0).

This patch is only doing the export so far, but I can do the import if this first part is accepted.

I have not found a suitable ontology for writing that in OPML, so I suggest using <outline ttrss:ttl="123"> with <opml xmlns:ttrss="http://tt-rss.org/">.

The TTL parameter exists in RSS 2.0 but without a namespace.
Another option is http://purl.org/rss/1.0/modules/syndication/ but not very convenient to use.
Other suggestions welcome.