Fix parsing of nested links

Enter event for `LabelText` should be inserted before existing events at the index in order
to have correct nesting. Otherwise nested elements could have Enter event first and that
would result in incorrect nesting in the tree when converting to AST.

Closes GH-73.
Closes GH-50.

Author: sheremetyev

src/construct/label_end.rs

@@ -752,7 +752,7 @@ fn inject_labels(tokenizer: &mut Tokenizer, labels: &[Label]) {
         // Though: if this was what looked like a footnote, but didn’t match,
         // it’s a link instead, and we need to inject the `^`.
         if label.start.1 != label.end.0 || !caret.is_empty() {
-            tokenizer.map.add(
+            tokenizer.map.add_before(
                 label.start.1 + 1,
                 0,
                 vec![Event {

tests/link_resource.rs

@@ -1,5 +1,5 @@
 use markdown::{
-    mdast::{Link, Node, Paragraph, Root, Text},
+    mdast::{Image, Link, Node, Paragraph, Root, Text},
     to_html, to_html_with_options, to_mdast,
     unist::Position,
     CompileOptions, Options,
@@ -511,5 +511,27 @@ fn link_resource() -> Result<(), String> {
         "should support link (resource) as `Link`s in mdast"
     );
 
+    assert_eq!(
+        to_mdast("[![name](image)](url)", &Default::default())?,
+        Node::Root(Root {
+            children: vec![Node::Paragraph(Paragraph {
+                children: vec![Node::Link(Link {
+                    children: vec![Node::Image(Image {
+                        alt: "name".into(),
+                        url: "image".into(),
+                        title: None,
+                        position: Some(Position::new(1, 2, 1, 1, 16, 15)),
+                    }),],
+                    url: "url".into(),
+                    title: None,
+                    position: Some(Position::new(1, 1, 0, 1, 22, 21)),
+                }),],
+                position: Some(Position::new(1, 1, 0, 1, 22, 21)),
+            }),],
+            position: Some(Position::new(1, 1, 0, 1, 22, 21))
+        }),
+        "should support nested links in mdast"
+    );
+
     Ok(())
 }