Add a --one-off / --command CLI option

[mildbyte]

Running e.g. ./seafowl --one-off "CREATE TABLE ..." should, instead of starting a server, just execute a single command against the context and exit. This is going to be used when baking a dataset into a Docker image, e.g.:

RUN \
	wget https://some-csv.com/file.csv -o file.csv && \
	seafowl --one-off \
		"CREATE EXTERNAL TABLE t LOCATION 'file://file.csv'; \
		CREATE TABLE t_frozen AS SELECT * FROM t;" && \
	rm file.csv