generate: don't alter # line inside fenced code block

Previously, `configlet generate` assumed that every line beginning
with a '#' character inside a concept `introduction.md` is a header.
For our purposes, the most common exception to that idea is probably
a comment inside a fenced code block. With this commit, we no longer
add a '#' to such a line.

The markdown handling here is rudimentary, but it may be good enough to
avoid a full markdown parser for `configlet generate`.

But this doesn't support other blocks, like HTML blocks.

src/generate/generate.nim

@@ -20,10 +20,17 @@ func alterHeaders(s: string): string =
   if i < s.len and s[i] == '#' and i+1 < s.len and s[i+1] == ' ':
     i += s.skipUntil('\n', i)
   # Demote other headers
+  var inFencedCodeBlock = false
   while i < s.len:
     result.add s[i]
-    if s[i] == '\n' and i+1 < s.len and s[i+1] == '#':
-      result.add '#'
+    if s[i] == '\n':
+      # When inside a fenced code block, don't alter a line that begins with '#'
+      if i+1 < s.len and s[i+1] == '#' and not inFencedCodeBlock:
+        result.add '#'
+      elif i+3 < s.len and s[i+1] == '`' and s[i+2] == '`' and s[i+3] == '`':
+        inFencedCodeBlock = not inFencedCodeBlock
+        result.add "```"
+        i += 3
     inc i
   strip result
 

tests/test_generate.nim

@@ -21,7 +21,10 @@ proc testGenerate =
 
         The quick brown fox jumps over a lazy dog.
 
-        The five boxing wizards jump quickly.
+        ```nim
+        # This line is not a header
+        echo "hi"
+        ```
 
         ## Header 4
 
@@ -45,7 +48,10 @@ proc testGenerate =
 
         The quick brown fox jumps over a lazy dog.
 
-        The five boxing wizards jump quickly.
+        ```nim
+        # This line is not a header
+        echo "hi"
+        ```
 
         ### Header 4