Add ots sanitization during build steps

[fred-wang]

See #12

If the font are rejected by OTS, they so will they be in some browsers when they are used as web fonts.

Probably we should add a step in the build script that run ots-sanitize on the otf input files.

[norbusan]

@fred-wang I can work on that. Do you intend to run ots-santize on the otf files, or on the generated woff(2) files? AFAIS it can run on the woff files.

[fred-wang]

@fred-wang I think we should do it on the otf files provided by font designers at least, so we can report any issues to them. In theory, WOFF is only lossless compression and WOFF2 compression with "minor tweaks" so the generated files should remain valid (I'm fine with performing a second validation after conversion, although I'm not sure if that will be very useful).