imp(macros.rs): Added write_nspaces macro (a new version of write_spaces)

`write_nspaces` has three differences with `write_spaces`

  1. Accepts arguments with attribute access (such as self.writer)
  2. The order of the arguments is swapped to make the writer the first
      argument as in other write related macros.
  3. Does not use the `write!` macro under the hood but rather calls
      directly `write`

I have chosen to put the function under a new name to avoid backwards
compatibility problem but it might be better to migrate everything to
`write_nspaces` (and maybe rename it `write_spaces`)

src/macros.rs

@@ -586,6 +586,19 @@ macro_rules! write_spaces {
     })
 }
 
+// Helper/deduplication macro for printing the correct number of spaces in help messages
+// used in:
+//    src/args/arg_builder/*.rs
+//    src/app/mod.rs
+macro_rules! write_nspaces {
+    ($dst:expr, $num:expr) => ({
+        debugln!("macro=write_spaces!;");
+        for _ in 0..$num {
+            try!($dst.write(b" "));
+        }
+    })
+}
+
 // convenience macro for remove an item from a vec
 macro_rules! vec_remove {
     ($vec:expr, $to_rem:expr) => {