initial crack at a block-level figure parser

I didn't like the other figure + figcaption parsers, they either assumed
a lot about usage (e.g. images only), or they were inline parsers that
either wrapped the figure in a paragraph tag (which is incorrect syntax)
or did span trickery (annoying)

so, this handles images and maybe other things, and does things properly
with figures as their own blocks. incomplete but it works with my
images, and should allow for looping (for multi-line content) in the
future?

tests/functional_markdown_tests.py

@@ -37,3 +37,16 @@ def test_invalid_graphviz_is_not_rendered():
     response = client.get('/test-invalid-graphviz')
     assert response.status_code == 500
     assert b'INTERNAL SERVER ERROR' in response.data
+
+
+def test_figures_are_rendered(client):
+    """Test that a page with my figure syntax renders as expected."""
+    response = client.get('/figures')
+    assert response.status_code == 200
+    assert (b'<figure class="right"><img alt="fancy captioned logo" src="bss-square-no-bg.png" />'
+            b'<figcaption>this is my cool logo!</figcaption></figure>') in response.data
+    assert (b'<figure><img alt="vanilla captioned logo" src="bss-square-no-bg.png" />'
+            b'<figcaption>this is my cool logo without an attr!</figcaption>\n</figure>') in response.data
+    assert (b'<figure class="left"><img alt="fancy logo" src="bss-square-no-bg.png" />'
+            b'<span></span></figure>') in response.data
+    assert b'<figure><img alt="just a logo" src="bss-square-no-bg.png" /></figure>' in response.data