SWISH::Prog notes
============================= 

base class - keep it simple
SWISH::Prog->new = 

    indexer handle
    UA handle
    fetch() method returns SWISH::Prog::Doc object, reads doc into buffer, headers, etc.
    index() method hands to indexer and can take either string or Doc object
    filter() - takes a Doc object, called in index()
    ok() - takes a Doc object, called in index()
    
SWISH::Prog::Doc methods

each method corresponds to a Header line, plus content()
url()
modtime()
mime() / type() (alias-> mime)  -- use File::Type
parser() - use parser map
content()

as_string() returns object as scalar string, ready for indexer

print $Doc should stringify same as as_string()

use overload
  (
   '""' => \&as_string,
   fallback => 1,
  );