This resolves a bunch of issues, detailed in #14. But basically, I was
assuming users would never specify an absolute path in `folder`. But
it turns out this was natural for at least one user: they specified
the full path there. And it actually works! Somehow the code was
finding the right path for the folder. But the problem with that is
that the `prefix` (the `mailbox` setting) would get created
anyways.

In a more extreme case, a user could possibly not specify a `mailbox`
at all and manually set a `folder` per feed, then would get a
`~/Mailbox` created even if *no* `folder` would reference this
folder (because that is the default "prefix").

The fix is surprisingly simple: just check if the provided folder is
an absolute path, and just prepend the prefix if it's not absolute.

We also need to create directories based on the resulting folder path,
not the prefix, in case we just don't use the prefix.

This is an API change, as the (undocumented) semantics of the
configuration file get changed.

Closes: #14