ox-md: Use TeX-style $ math wrapping

* lisp/ox-md.el (org-md-latex-environment, org-md-latex-fragment):
These two new filters wrap LaTeX maths in $ / $$ TeX-style notation.
While ox-md endeavours to adhere to the original Markdown specification,
and not any particular variant, the original specification does not deal
with LaTeX fragments at all, and so this change does not reduce how
faithfully the original specification is followed.
There is a major upside to this though. Of the many Markdown variants
that have emerged, those that support LaTeX very often exclusively
support TeX style notation.  This change thus improves the utility of
the Markdown export for many use cases, and deviates no more from the
original specification that the current method.

lisp/ox-md.el

@@ -101,6 +101,8 @@ The %s will be replaced by the footnote reference itself."
 		     (italic . org-md-italic)
 		     (item . org-md-item)
 		     (keyword . org-md-keyword)
+                     (latex-environment . org-md-latex-environment)
+                     (latex-fragment . org-md-latex-fragment)
 		     (line-break . org-md-line-break)
 		     (link . org-md-link)
 		     (node-property . org-md-node-property)
@@ -461,6 +463,35 @@ channel."
     (_ (org-export-with-backend 'html keyword contents info))))
 
 
+;;;; Latex Environment
+
+(defun org-md-latex-environment (latex-environment contents info)
+  "Transcode a LATEX-ENVIRONMENT object from Org to Markdown.
+CONTENTS is nil.  INFO is a plist holding contextual information."
+  (when (plist-get info :with-latex)
+    (let ((latex-frag (org-remove-indentation
+                       (org-element-property :value latex-environment)))
+          (label (org-html--reference latex-environment info t)))
+      (if (org-string-nw-p label)
+          (replace-regexp-in-string "\\`.*"
+                                    (format "\\&\n\\\\label{%s}" label)
+                                    latex-frag)
+        latex-frag))))
+
+;;;; Latex Fragment
+
+(defun org-md-latex-fragment (latex-fragment contents info)
+  "Transcode a LATEX-FRAGMENT object from Org to Markdown.
+CONTENTS is nil.  INFO is a plist holding contextual information."
+  (when (plist-get info :with-latex)
+    (let ((frag (org-element-property :value latex-fragment)))
+      (cond
+       ((string-match-p "^\\\\(" frag)
+        (concat "$" (substring frag 2 -2) "$"))
+       ((string-match-p "^\\\\\\[" frag)
+        (concat "$$" (substring frag 2 -2) "$$"))
+       (t frag))))) ; either already $-deliminated or a macro
+
 ;;;; Line Break
 
 (defun org-md-line-break (_line-break _contents _info)