Add config file, for setting default parameters

[eklitzke]

At the minimum, we want to support setting a default sample rate (same as the -r option) and sample time (same as the -s option).

Ideally this could be done using the XDG spec, so that it would be something like:

• First read /etc/pyflame/config.yaml
• Merge with ~/.config/pyflame/config.yaml

That way a site operator can update the default settings globally. It looks like there are a few C and C++ libraries that implement basic XDG support.