input missing copyright alias from stdin

[Peter Johansson]

Currently, if there is a missing copyright alias, svndigest will give a warning

svndigest: warning: no copyright alias found for `author'

and then use the raw user name. It wouldn't be too hard to stop and ask for the alias, so the user simply can input the name rather than edit the config file and restart the run.

[Peter Johansson]

I don't like use interaction. Since files are parsed updated file by file rather than all files parsed before updating, this is less annoying nowadays. Also the cache makes the problem virtually non-existing.