Load all (fixes #4)

I did this based on what had been done on the spikes.  There is some redundancy with only slight variations in behavior.

build_from_file: takes exact path and preprocessor (though it would usually be based on the config)
load: builds relative to contracts_path, adds '.json', no preprocessor support
load_all: builds multiple contracts relative to contracts_path, no preprocessor support

Obviously some cleanup is necessary, but what?  Personally I don't use preprocessors now that postprocessors exist.  Even if we keep it, we can use the Pacto.configuration.