Support arbitrary overrides of MET config values #30

Since wxvx offers only limited control of the MET config via its own YAML configuration language, preferring to support a curated set of common-use-case configs, accept a block of YAML data, under a designated key, to be used to override (or supplement) computed (or default) values. This may require extending the wxvx.metconf module to support all valid MET config syntax, so that any overrides applied to the config dict can be translated into MET config syntax.