Create exercism directory on configuration

I wasn't really sure where to place the logic to create the directory. Initially, I had planned to put it under `configure()` but that gets called when reading the contents of the file. It also seems like errors coming from `configure()` don't get checked but this is an error we'd generally want to display.

I decided to go for creating an exported function of Config and calling it before writing the file so that the file does not have a directory that could not be created. Let me know if anything jumps out. 

Fixes #122