Some changes suggested/provided by Eric Raymond:

Some changes suggested/provided by Eric Raymond:

- explain seekable
- when seekable==1, test fp.tell() and set it to 0 if that fails
- support overridable method iscomment(line) to weed out comments
- check for unread() method on file object before trying to seek

And one of my own:

- Add a get() method which behaves like a dictionary's get(); this is
actually implemented by giving getheader() an optional second argument
to specify the default, and aliasing get to getheader.