Define a demo directory

If you are not logged in, there's no way to know where you want to
download sample files to. A subdirectory under the current working
directory seems reasonable, and it's easier to move the result around or
delete it (because each language gets a subdirectory under that).
Optionally allow demo when not logged in.

If no config is found, create a demo directory in the current working
directory, and download the assignments to it.

This needs refactoring, but I don't know Go well enough to do that yet.
Merge pull request #11 from msgehard/demo-without-login

`exercism demo` should not require login