- [ ] `add` and `edit` require an empty line at the end of the file (this is brittle)
- [x] `parse_book` expects to hit an '=' on any given line, but if we're gonna support
       long lines (e.g. quotes) then we'll want to check for '\n' as well
- [x] `get_string_field` and `get_number_field` should return the pointer to the field
       so we can actually use it in `search` and `parse_book`
- [ ] fix unknown fields parsing and editing (capture, don't just skip)
- [ ] when an int can't be parsed, it reports it for every line in the file, should only report it once