export completed
Merge branch 'export'
add `Maybe<T>`
handle invalid minos
add sample wrong.txt
Merge branch 'example' into invalid_mino
Merge branch 'invalid_mino'
cleanup 14

combine export2 with export_
add newline
separate maybe.h from header.h
cleanup 15

combine `get_mino_data` with `import_`
rename `header.h` to `status.h`
make verbose if-else shorter