Input file is a list of links.  Each link can optionally be preceded by
a suggested prefix.  The digest already has some default prefixes it
will use if you don't override them:

* u - unknown (HINT: it's a good idea to use 'b' for blog)
* m - haskell-cafe
* s - stackoverFlow
* r - reddit

An example of what the input file might look like, see [test.links](test.links):

    http://www.reddit.com/r/haskell/comments/n2mws/ghc_commit_allow_the_number_of_capabilities_to_be/
    http://www.reddit.com/r/haskell/comments/n7iv1/haswell_processor_hardware_transactional_memory/
    t http://www.well-typed.com/Hal6/
    https://github.com/polux/hpuns
    p https://groups.google.com/d/topic/parallel-haskell/8YelldrF0QI/discussion

You would run

    mk-digest -f test.links