make api compatible with code using legacy library

[eyaler]

i just upgraded a few sketches which use ml5 handpose and facemesh from 0.12.2 to next-gen alpha4, due to the considerable freezes on model load. the new preload is much more bearable. however there are many differences in the api of these models, also besides the loading, and part of which may be due to upstream changes. to support the multitude of legacy code, and allow it to transition to next-gen and benefit from smoother loading, i suggest to implement api's compatible with the legacy library, in parallel with the new api's. if this approach is acceptable, i can work on a PR for handpose and facemesh.

[shiffman]

Hi @eyaler, we are planning to make this version of ml5.js 1.0 and thus incorporate breaking changes to the API. The previous version of the library and its documentation will be archived and can still be used! However, I think it will require too much maintenance and complexity to try to include legacy functions and naming conventions in the new codebase! We'll be trying to keep things backwards compatible as we move forward, but these models are all new!