Improve models builders

[n1t0]

• from_files now returns a builder, and cannot fail
• All possible failures are handled during the build phase of each builder
• This also moves any I/O to the build phase. This behavior seems more intuitive: we first construct our builders with all their respective configurations and then finally build at the end, doing the real work only then.
• This simplifies both python and node bindings where we don't need to have any default values.
• Add the continuing_subword_prefix option to both bindings for WordPiece.

[epwalsh]

I like this! I think it's definitely a good convention.

[Pierrci]

👍