Add support for config dotfiles

To close #5.

The config file can include the host name (overridable using the -H switch).

I was unsure whether to retain environment variable support too - I eventually decided to remove it to avoid confusion (as per @jmathai's comment: https://github.com/photo/openphoto-python/issues/5#issuecomment-3609710). Let me know if you think I should put it back.