Implement basic `prefix` command

[jamesaoverton]

The prefix command is used to add prefixes to the prefix table. Eventually I would like to be able to read prefixes from Turtle files, RDFXML files, JSON-LD contexts, etc. For now I would be happy to read prefixes from a TSV file:

ldtab prefix test.db prefix.tsv

If no input TSV is provided, we should print the prefixes.

I made a start on tests in tests/prefix/*