[Build] Added local-overrideable Build.EDITOR, defaults to "mvim -p"
[Build] Added local-overrideable Build.EDITOR, defaults to "mvim -p"

- Can use to implement 'rogo edit' in BuildLocal.rogue.
- Example:
    routine rogo_edit
      local cmd = Build.EDITOR
      cmd += @| Test.rogue
      cmd += @| Source/Libraries/Standard/Standard.rogue
      cmd += @| Source/RogueC/RogueC.rogue
      Build.run( cmd )
    endRoutine
- Can override EDITOR in Local.mk:
  EDITOR=code

Plasmaworks/Rogue,Plasmaworks/Rogue,AbePralle/Rogue,Plasmaworks/Rogue,AbePralle/Rogue,Plasmaworks/Rogue