Add MD014 with tests.

test/code_block_dollar.md

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+The following code block shouldn't have $ before the commands:
+
+    $ ls {MD014}
+    $ less foo
+
+    $ cat bar
+
+However the following code block shows output, and $ can be used to
+distinguish between command and output:
+
+    $ ls
+    foo bar
+    $ less foo
+    Hello world
+
+    $ cat bar
+    baz
+
+The following code block uses variable names, and likewise shouldn't fire:
+
+    $foo = 'bar';
+    $baz = 'qux';
+
+The following code block doesn't have any dollar signs, and shouldn't fire:
+
+    ls foo
+    cat bar
+
+The following (fenced) code block doesn't have any content at all, and
+shouldn't fire:
+
+```
+```