Add a default config file location for Windows

[kivikakk]

See https://github.com/DanielKeep/cargo-script/blob/614e60e5932e218ebff1e471303eb0d59870d03b/src/platform.rs#L309-L363 for an example of getting the roaming app data folder.

[coolaj86]

I'd argue that it's better to have a single location ~/.config/comrak/config that can be relied on cross-platform rather than many different locations.

Obviously people with Roaming Profiles on an Active Directory domain that switch between various work computers on a whim do exist, but I imagine that once you've narrowed that down to people who enjoy the command line with Unix tools... they're probably used to solving that particular problem in a more Unix-y way.

Likewise, I don't think too many developers on OS X prefer to have their unix tools saving to ~/Library/Application Support rather than the more familiar ~/.config.