[WIP] Global Configuration and current-course

[thinkmorestupidless]

The aim of this PR is to add just enough functionality to remove the need to pass a repository url with everything cmtc command.

• Add support for a CMT_HOME directory (by default ~/.cmt
• Add a default global cmt configuration file
• Add a new configure command to cmtc tool for creating the above on a local machine
• Add a check before executing each cmtc command to ensure configuration has taken place and that a current-course is set in the configuration file
• Add a new set-current-course command to ctmc to allow a user to point cmtc at a course on their local filesystem.