Add an artifacts proxy to GitLab Pages

Edit app_config.go to add an ArtifactServer attribute to the appConfig struct that is the url.URL type.

Add new command line flag for the setting of the artifact-server option, then adjust the configFromFlags function to read the new artifact-server flag and include it while building the programs appConfig value.

Creation of new artifact struct and related functions with within artifact.go that is used to build url’s from the artifact-server flag value, values from within the host name, and the subsequent http requests to this program.

Integration of this artifact type within app.go’s serveContent function to proxy requests where applicable. Create unit tests accordingly.

Closes #78 (closed)