Adds configuration loading from a file located at either
"/etc/gemma/config.lisp" or a path determined via the `GEMMA_CONFIG`
environment variable.
The configuration file can contain any number of deftask forms and a
single config form which determines the location at which Gemma stores
its data and also the port on which it should listen.
Uses the cl-prevalence system to store tasks on disk. The storage
location is either relative to the working directory in which the
system is started or determined (with priority) by the environment
variable `GEMMA_DATA_DIR`.
