Allow per-map setting of imagery.generic.*_expires

[jidanni]

Please allow per-map setting of imagery.generic.*_expires.

Otherwise even if just one map on one site needs a different setting,
not only all the maps on that site are forced to use it,
every map on every site is too.

Compare
​http://www.offline-maps.net/en/help/maps/stored/custom#advanced_example 's
per-map <update-delay> tag.

[jidanni]

​http://www.alpinequest.net/en/help/v2/maps/on-demand-create has even more features.

[jidanni]

P.S., imagery.generic.minimum_expires has no documentation yet.
When creating the documentation do mention how it interacts with the tags servers send. E.g., as seen in
​https://help.openstreetmap.org/questions/62203/how-to-change-josm-imagerygenericminimum_expires-but-just-for-one-imagery
Say if it overrides those tags and which.