sentencepiece bpe compatible tokenizer (#252)

* potential out of bounds read

* fix quantize

* style

* Update convert-pth-to-ggml.py

* mild cleanup

* don't need the space-prefixing here rn since main.cpp already does it

* new file magic + version header field

* readme notice

* missing newlines

Co-authored-by: slaren <2141330+slaren@users.noreply.github.com>