Add test for meta directive and add visitor functions for latex and text writers.

tests/root/markup.txt

@@ -7,6 +7,10 @@ Testing various markup
 
 .. contents:: TOC
 
+.. meta::
+   :author: Me
+   :keywords: docs, sphinx
+
 
 Admonitions
 -----------

tests/test_build.py

@@ -54,6 +54,10 @@ HTML_XPATH = {
         ".//dt[@id='test_autodoc.function']/em": '**kwds',
         ".//dd": 'Return spam.',
     },
+    'markup.html': {
+        ".//meta[@name='author'][@content='Me']": '',
+        ".//meta[@name='keywords'][@content='docs, sphinx']": '',
+    },
 }
 
 class NslessParser(ET.XMLParser):