allow shell command on new items only

[anarcat]

feed2imap allows users to customize commands to download and filter complete feeds, which is useful for fixing up feeds or dealing with exotic configurations. but it would be really useful if we could have commands that would be ran on individual items when they are updated. this could be hijacked to transform feed2imap into a crude podcast client, for example, as a simple wrapper around wget could be implemented to snatch the <link> tag from an item.

i tried to look into implementing this and it seemed like i first needed to modify the ConfigFeed class to add new settings (and maybe somewhere else). then the main fetch loop in Feed2Imap::initialize would need to be changed to call those commands. i don't feel comfortable enough with the code to do this myself at this stage, but if there's interest, i may give it a try.