Autoflush

I've added a auto flush function to miniflux to flush read messages in history.
It prevent the db to grow too much. 

You can set up the number of days to keep in history (1 new field in the db)

The autopurge is called in the index.php, on action=history and Router\notfound(function()

an other way would have to add 1 if/select case in index.php,  on action :
if unread or history then autoflush