Re-scan of MBTiles directory after adding new files

[RoeeDev]

When I run martin with a config file with a directory of MBTiles configured it all works great, but when I try to add new files it doesn't re-scan the directory and I need to re-run martin for it to publish them. Is there a way to make martin publish the new MBTiles files as soon as they are added in the directory?

[nyurik]

Duplicate of #288, lots of requests for this, some work is being done in #1179