Add a configuration file to store defaults

[Gosha Tcherednitchenko]

I've noticed that I tend to use Chrono with some specific defaults that I keep repeating. Rather than using shell aliases, it would be good to centralise defaults in a configuration file.

Examples of defaults I'd personally use:

• Show the report for the current week by default instead of the current day
• Start the week on a Monday (see #12)
• Set default tags for projects
• And so on

Thinking further, it would be great to support per-project configuration files, so that running chrono start in a directory that contains a .chronorc file would automatically start a new frame for the correct project with the correct tags.