Create binaries automatically from tags

[grovesNL]

We would like to use the same method as trust to generate binaries and attach to GitHub releases (which are just tags). We can use this both for fixed releases and "latest" rolling releases in which the tag is simply moved (like https://github.com/KhronosGroup/SPIRV-Tools/releases/tag/master-tot).

To get started with .travis.yml changes:

• Use make package.zip to generate the package for tag builds only (probably need to rename these packages to include OS name before uploading)
• Add deploy to .travis.yml to deploy the created package, see trust's .travis.yml for an example
• We will need to use an encrypted API key/OAuth token in order to perform the upload
  • Should we use a special GitHub user which only has access to portability to do this? Otherwise we would need to tie it to one of our personal accounts and just limit permissions.

The appveyor.yml setup is similar though I haven't looked into it as much yet.

[kvark]

This was implemented in a series of commits, huge thanks to @grovesNL for championing this!