Proof of concept to turn entries into mails

Does not handle extra fields for extra rss-info yet (useful for filtering)
and does not do anything useful with the mails yet.
Add special headers to mails for feed, item and id.

A bigger refactoring was needed for this as the Mail needs to be constructed
manually now. It also avoids the need for one unwrapping of an IO. The mails
now contain additional fields with details about the entry and feed.
Introduce a nice From indicating author of the entry

Since usually the author is interesting for readers, lets indicate that
in the From. Also add the feed title so with multiple feeds its clear where
a mail belongs to.
Refactoring for a bit nicer code.
Add Date header using the entry's date

This makes it easy to sort feed entries based on their publishing date.
Currently the Feed library does not support modified etc. entries.
Introduce commandline arguments

Currently only debug does something useful, it prints out all mails
generated from the given feed to stdout. This is intended for easy debugging
of the code while developing the rest of the features.