update default exercism assignment path to ~/exercism

Previously the default path (if none was provided by the user) when logging in defaulted to a folder called exercism in the current working directory. Now the default path is relative to the user's home directory. If the user requests a demo the demo directory will also be relative to the home directory.

Feedback welcome!

Closes #96
This overwrites the package-level variable, which seems magical and surprising to me.

I'd rather use a function (not exported) that sets the value, for the sake of clarity.