Fix indented code blocks (#269)

features/codeblock_indented.feature

@@ -0,0 +1,23 @@
+Feature: indented codeblock
+
+  Background:
+    Given file "1.md" with content:
+      """
+      # Title
+      [Two](2.md)
+
+      - point 1
+
+        ```go
+        result := map[^0]
+        ```
+      """
+    And file "2.md" with content:
+      """
+      # Two
+      [One](1.md)
+      """
+
+  Scenario: check
+    When checking
+    Then it finds no issues

features/codeblock_toplevel.feature

@@ -0,0 +1,21 @@
+Feature: top-level codeblock
+
+  Background:
+    Given file "1.md" with content:
+      """
+      # Title
+      [Two](2.md)
+
+      ```go
+      result := map[^0]
+      ```
+      """
+    And file "2.md" with content:
+      """
+      # Two
+      [One](1.md)
+      """
+
+  Scenario: check
+    When checking
+    Then it finds no issues

src/database/line.rs

@@ -12,6 +12,7 @@ static MD_RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"(!?)\[[^\]]*\]\(([^)]*)\)"
 static A_HTML_RE: Lazy<Regex> = Lazy::new(|| Regex::new(r#"<a href="(.*)">(.*)</a>"#).unwrap());
 static IMG_HTML_RE: Lazy<Regex> = Lazy::new(|| Regex::new(r#"<img src="([^"]*)"[^>]*>"#).unwrap());
 static FOOTNOTE_RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"\[\^([\w-]+)\](:?)").unwrap());
+static CODEBLOCK_RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"^\s*```").unwrap());
 
 impl Line {
   /// appends all footnote definitions and references to the given result structure
@@ -44,7 +45,7 @@ impl Line {
 
   /// indicates whether this line is the beginning or end of a code block
   pub fn is_code_block_boundary(&self) -> bool {
-    self.text.starts_with("```")
+    CODEBLOCK_RE.is_match(&self.text)
   }
 
   /// populates the given accumulator with all links and images in this line