support checking validity of configuration during initialization

[alecjohnson]

A general principle of maintainable code is that error messages should be issued as soon as the violation of an assumed condition can be efficiently detected.

Currently configuration options are not checked for validity when they are read in, and only a small number of checks are made later on. Tightly and cleanly restricting configuration options allows us to guarantee backward compatibility in how the configuration file drives behavior. We should validate configuration options on startup and immediately exit with an informative error message upon detecting invalid options.

Note that appropriate resolution of this issue assumes that issue #64 is also resolved.