Added an `only` directive that can selectively include text

based on enabled "tags".  Tags can be given on the command
line.  Also, the current builder output format (e.g. "html" or
"latex") is always a defined tag.

tests/root/conf.py

@@ -47,6 +47,8 @@ value_from_conf_py = 84
 coverage_c_path = ['special/*.h']
 coverage_c_regexes = {'cfunction': r'^PyAPI_FUNC\(.*\)\s+([^_][\w_]+)'}
 
+# modify tags from conf.py
+tags.add('confpytag')
 
 from sphinx import addnodes