* A flag `--keep-going / -k' to keep building goals if one fails, as

much as possible.  (This is similar to GNU Make's `-k' flag.)

* Refactoring to implement this: previously we just bombed out when
  a build failed, but now we have to clean up.  In particular this
  means that goals must be freed quickly --- they shouldn't hang
  around until the worker exits.  So the worker now maintains weak
  pointers in order not to prevent garbage collection.

* Documented the `-k' and `-j' flags.

src/libmain/shared.cc

@@ -137,6 +137,8 @@ static void initAndRun(int argc, char * * argv)
         }
         else if (arg == "--keep-failed" || arg == "-K")
             keepFailed = true;
+        else if (arg == "--keep-going" || arg == "-k")
+            keepGoing = true;
         else if (arg == "--max-jobs" || arg == "-j") {
             ++i;
             if (i == args.end()) throw UsageError("`--max-jobs' requires an argument");