Adds format functionality

[1wheel]

Hey @j6k4m8, this package has been super useful. I sat down to add the ability save files #15 and ended up tweaking a bunch of things:

• Adds a format function
• Only expose the parse and format functions. This avoids the need for load error handling File failing to load.  #9; there's also not any need for the class wrapper.
• Converts to a module so the same code can run in node or the browser without bundling.
• parse throws errors if it encounters unsupported npy options
• parse reads both bytes of header length and can handle files with larger headers
• 2 spaces instead of 4 : p

That's a bunch of breaking changes—feel free to only grab the things you want!