More elegant fix for #383.

The problem arose as follows. The input was

```
*****Hello*world****
```

We have an `openers_bottom` table that limits how far back you have to
look for an opener. It is indexed to the type of delimiter (`_` or `*`)
and the length of the closing delimiter mod 3.  So after we fail to match
the opener `*****` to `*`, we set the openers_bottom for `(*, 1)` to the
location of `*`, effectively removing the `*****` as a possible opener for
any run of `*`s with a length mod 3 of 1, including the final `****` in this
example. This procedure ignores the fact that the length mod 3
restriction only matters if one of the delimiters can be both an opener
and a closer.

To fix this problem, we index the `openers_bottom` table not just to
the type of delimiter and the length of the closing delimiter mod 3,
but to whether the closing delimiter can also be an opener.

src/inlines.c

@@ -645,7 +645,8 @@ static void process_emphasis(subject *subj, delimiter *stack_bottom) {
   delimiter *old_closer;
   bool opener_found;
   int openers_bottom_index = 0;
-  delimiter *openers_bottom[6] = {stack_bottom, stack_bottom, stack_bottom,
+  delimiter *openers_bottom[9] = {stack_bottom, stack_bottom, stack_bottom,
+                                  stack_bottom, stack_bottom, stack_bottom,
                                   stack_bottom, stack_bottom, stack_bottom};
 
   // move back to first relevant delim.
@@ -667,7 +668,8 @@ static void process_emphasis(subject *subj, delimiter *stack_bottom) {
         openers_bottom_index = 2;
         break;
       case '*':
-        openers_bottom_index = 3 + (closer->length % 3);
+        openers_bottom_index = 3 +
+		(closer->can_open ? 3 : 0) + (closer->length % 3);
         break;
       default:
         assert(false);