#863 extensible snapshots

[mgsloan]

• Extensible custom snapshots implemented - see Extensible snapshots #863. These allow you to define snapshots which extend other snapshots. Local file custom snapshots can now be safely updated without changing their name. Remote custom snapshots should still be treated as immutable.
• Custom snapshots now support ghc-options.
• Consolidates logic for ghc-options and flags (now mostly in Stack.Build.Source)

@borsboom Want to review this? I think it's pretty solid, but it could be valuable to get a review, particularly the changes in Stack.BuildPlan and Stack.Types.Config.