build: -pkg flag to specify package name (#150)

Allows override in the case where package name is incorrectly deduced from directory name.

Fixes #68
Fixes #147

tests/fixedbugs/issue68/issue68_test.go

@@ -0,0 +1,13 @@
+package custom
+
+import (
+	"testing"
+)
+
+//go:generate go run asm.go -pkg custom -out issue69.s -stubs stub.go
+
+func TestIssue68(t *testing.T) {
+	if got := Issue68(); got != 68 {
+		t.Fail()
+	}
+}