allow multiple config files

[Peter Johansson]

We should allow multiple config files. This implies that user can have one config file specific for the project that is analyzed (digested), but also a global config that contain settings that the user always want to use. These should not be two different kinds of files and absolutely not two kinds of Configuration classes. Rather what it means is that load(istream&) function in Configuration should be called twice.

I suggest the following schema

1. Create Configuration object
2. If $HOME/.svndigest/config exists load Configuration object from that file
3. If <root>/.svndigest/config exists load Configuration object from that file
4. If option --config-file is given and file exists load Configuration (if file does not exist exit)

In case several files are loaded and an option is set in multiple files, the latter file overrides the former one.

[Peter Johansson]

I am not sure about this ticket. Perhaps usability will not improve but only get more confusing. I move this to the future.