Expose config for problems host #145
Description
At the moment we only expose the config for the web api (to submit problems, etc), but not the problems api (where we get the actual READMEs and test suites). This is usually fine, since most people can simply use the website locally and point to the remote problem API.
This becomes a problem when people are running a completely separate installation of exercism with custom problems.
The configuration should optionally handle an additional argument to configure the problems API.
/cc @morphatic