Add flag to read buildNumber from .build.

By setting `<meta build-number=".build" />`, you can force Lime not
to calculate a build number from git or svn, but instead use the
default option of getting it from the .build file.

Since setting this value would previously have produced useless
build numbers, it's unlikely that this affects any existing builds.

tools/CommandLineTools.hx

@@ -1257,7 +1257,7 @@ class CommandLineTools
 			buildNumber = getBuildNumber_SVN(project, increment);
 		}
 
-		if (buildNumber == null)
+		if (buildNumber == null || buildNumber == ".build")
 		{
 			var versionFile = Path.combine(project.app.path, ".build");
 			var version = 1;