Use image description text as "alt", drop title

The current RecommonMark specification on images [0] says that:

    ![foo](/url "title")

should render as

    <p><img src="/url" alt="foo" title="title" /></p>

which means that "foo" should be the `alt` attribute, and "title" should
be the `title` attribute.

Currently, `recommonmark` will:

1. set the `alt` attribute to "title"
2. render "foo" as literal text following the image element.

Neither yields results in line with the RecommonMark standard, resulting
in the following when transformed to HTML:

    <p><img src="/url" alt="title" />foo</p>

While it might be surprising that `alt` is set to "title", the more
pressing issue is how the alt text becomes literal text within the
paragraph, typically not rendering well.

This commit instead makes `recommonmark`:

1. set the `alt` attribute to "foo"
2. drop "title" altogether since the `title` attribute is not supported
   in Docutils [1].

1 coincides with the specification, and 2 is in my mind the least
surprising solution within the capabilities of Docutils. The HTML will
now be:

    <p><img src="/url" alt="foo" /></p>

only differing in the missing `title` attribute when compared to the
specification.

[0]: https://spec.commonmark.org/0.28/#images
[1]: http://docutils.sourceforge.net/docs/ref/rst/directives.html#image

recommonmark/parser.py

@@ -178,8 +178,9 @@ class CommonMarkParser(parsers.Parser):
         img_node = nodes.image()
         img_node['uri'] = mdnode.destination
 
-        if mdnode.title:
-            img_node['alt'] = mdnode.title
+        if mdnode.first_child and mdnode.first_child.literal:
+            img_node['alt'] = mdnode.first_child.literal
+            mdnode.first_child.literal = ''
 
         self.current_node.append(img_node)
         self.current_node = img_node

tests/test_basic.py

@@ -232,7 +232,7 @@ class TestParsing(unittest.TestCase):
             <?xml version="1.0" ?>
             <document source="&lt;string&gt;">
               <paragraph>
-                <image alt="title" uri="/url">foo</image>
+                <image alt="foo" uri="/url"></image>
               </paragraph>
             </document>
             """

tests/test_sphinx.py

@@ -129,7 +129,7 @@ class GenericTests(SphinxIntegrationTests):
     def test_image(self):
         output = self.read_file('index.html')
         self.assertIn(
-            '<p><img alt="Example" src="image.png" />foo</p>',
+            '<p><img alt="foo" src="image.png" /></p>',
             output
         )