Binary upgrade code path

[snoyberg]

• Add a new option for the stack upgrade command, --binary, that will download the latest binary version from the Github release (--no-binary will also be available)
• When compiling official releases, have some flag that indicates that this is an official release
• When using an official binary release, --binary is on by default. Otherwise, it's off by default.

[Blaisorblade]

• Show the "official release" flag in stack --version, as suggested by Have verbose stack log indicate that it was built via cabal-install, and if so, which dependencies were used  #2222 (comment).

[borsboom]

Bumped priority of this in light of planning to stop maintaining Linux APT/YUM repos.