Validator and friends versioning policy

At the moment, we have glTF spec version `1.0.1`, with `WebGL 1.0.3` profile (this also implies `GLSL ES 1.0`).

First release version of Validator will obviously have version `1.0`. However numbering future versions could become a mess, because it's unclear should Validator's version be aligned with spec or not.

Some possible strategies:
- fork validation code each time;
- incorporate a version check into every validation function;
- drop support for old spec versions, or make something like a sliding window (like GPU drivers handle old hardware).

Ideas?

Start by reviewing the versioning problem and the three strategies listed in the issue, including compatibility with glTF 1.0.1, the WebGL 1.0.3 profile, and GLSL ES 1.0. Done means agreeing on and documenting a clear policy for Validator versioning and support for older spec versions.

Written by the indexing model from the issue text.