Add options for interop with external tools

[efskap]

@sehnsucht13 proposed an interesting idea to me the other day:

To make sefr usable by external tools such as editors, have a command line switch that takes a partial search term, outputs a list of suggestions, then exits. Then, the external tool would choose one of the suggestions and feed it back through a different switch, which would cause sefr to perform the search non-interactively.

The only thing to decide is how prefixes and their escaping is handled. Does the former switch output suggestions with the prefix? And in either case, are they escaped? (For example, in the demo gif, selecting a suggestion starting with wkt escapes it with a question mark, turning it into ?wkt to geojson, as wkt is already registered as a search engine prefix.