Add `list` and `open` commands; delete deprecated behavior

This release adds two new comminds:

• exercism list $LANGUAGE to display available exercises
• exercism open $LANGUAGE $PROBLEM to open the most recent iteration of the give problem in the browser.

It also adds host connectivity status to both the problems API and the submissions API, to help with tricky troubleshooting issues.

We've finally deleted some legacy stuff:

• We won't read the ~/.exercism.go file. If you want to use that, then set the EXERCISM_CONFIG_FILE environment variable.
• The login/logout commands are gone. Use the exercism configure command.
• The deprecated key names in the config have been removed. If you have trouble authenticating, then reconfigure using the exercism configure command.

There are also a couple of bugfixes:

• Expand '~' in config path to home directory
• Display problems that have not yet been submitted when fetching. This should avoid the support tickets where people are unable to move forward.