Cache for application version

[Łukasz Niemier]

Currently `mix.exs` is always evaluated when compiling project, even when it is only dependency. This has its positive side, but also one negative side: application version need to be hardcoded somewhere, either within `mix.exs` itself or in some file, commonly `VERSION`. However this has main disadvantage - keeping all of them in sync, especially when using SCM as version source. What I am proposing here is to add new function to Mix (I haven't found proper place for it yet) that will read version from some known metadata file (for example `hex_metadata.config` file) and if this file do not exist, then it will fall back to provided callback function, ex.

def project do
  [
    # …
    version: try_cached_version(&version/0),
    #…
  ]
end

def version do
  case System.cmd("git", ~w[describe]) do
    {version, 0} -> version
    _ -> "0.0.0-dev"
  end
end

This would allow users to generate version via their project version using SCM of their choice and would not fail when building project as dependency.