Added MD_SPAN_A_DETAIL.is_autolink (#181)

This allows the processor to tell whether an <A> tag is the result of
an autolink, and customize its output. For example, I want to emit an
autolink of an image URL as an <IMG> tag, and an autolink of a YouTube
URL as a video embed.

src/md4c.c

@@ -4105,7 +4105,7 @@ md_analyze_link_contents(MD_CTX* ctx, const MD_LINE* lines, int n_lines,
 
 static int
 md_enter_leave_span_a(MD_CTX* ctx, int enter, MD_SPANTYPE type,
-                      const CHAR* dest, SZ dest_size, int prohibit_escapes_in_dest,
+                      const CHAR* dest, SZ dest_size, int is_autolink,
                       const CHAR* title, SZ title_size)
 {
     MD_ATTRIBUTE_BUILD href_build = { 0 };
@@ -4117,10 +4117,10 @@ md_enter_leave_span_a(MD_CTX* ctx, int enter, MD_SPANTYPE type,
      * MD_SPAN_IMG_DETAIL are binary-compatible. */
     memset(&det, 0, sizeof(MD_SPAN_A_DETAIL));
     MD_CHECK(md_build_attribute(ctx, dest, dest_size,
-                    (prohibit_escapes_in_dest ? MD_BUILD_ATTR_NO_ESCAPES : 0),
+                    (is_autolink ? MD_BUILD_ATTR_NO_ESCAPES : 0),
                     &det.href, &href_build));
     MD_CHECK(md_build_attribute(ctx, title, title_size, 0, &det.title, &title_build));
-
+    det.is_autolink = is_autolink;
     if(enter)
         MD_ENTER_SPAN(type, &det);
     else

src/md4c.h

@@ -284,6 +284,7 @@ typedef struct MD_BLOCK_TD_DETAIL {
 typedef struct MD_SPAN_A_DETAIL {
     MD_ATTRIBUTE href;
     MD_ATTRIBUTE title;
+    int is_autolink;            /* nonzero if this is an autolink */
 } MD_SPAN_A_DETAIL;
 
 /* Detailed info for MD_SPAN_IMG. */