issue doxygen#11420 Triple underscores in Markdown mode not supported

Support the `_` analogous to `*`

Author: albert-github

src/markdown.cpp

@@ -84,7 +84,7 @@ enum class ExplicitPageResult
 #define extraChar(c) \
   (c=='-' || c=='+' || c=='!' || \
    c=='?' || c=='$' || c=='@' || \
-   c=='&' || c=='*' || c=='%' || \
+   c=='&' || c=='*' || c=='_' || c=='%' || \
    c=='[' || c=='(' || c=='.' || \
    c=='>' || c==':' || c==',' || \
    c==';' || c=='\'' || c=='"' || c=='`')