Add prefixes to output with --add-prefix and --add-prefixes

[beckyjackson]

See discussion in #457

@jamesaoverton -ap and -AP work fine but if you'd like me to remove them and have no short option, I can do that.

usage: robot [command] [options] <arguments>
 --add-prefix <arg>     add prefix 'foo: http://bar' to the output
 --add-prefixes <arg>   add JSON-LD prefixes to the output

---

The --add-prefix and --add-prefixes options allows you to specify prefix mappings in the same way as --prefix and --prefixes:

robot --noprefixes --add-prefix "foo: http://foo#" \
  export-prefixes --output results/foo.json

robot --noprefixes --add-prefixes foo.json \
  --add-prefix "bar: http://bar#" --add-prefix "baz: http://baz#" \
  export-prefixes

The difference is that the --prefix/--prefixes options do not include the new prefix in the header of the output ontology, whereas --add-prefix/--add-prefixes options do, for example in Turtle:

@prefix foo: <http://foo#> .