add options for sub and sup tags

fixes #44

README.rst

@@ -84,6 +84,11 @@ strong_em_symbol
   *emphasized* texts. Either of these symbols can be chosen by the options
   ``ASTERISK`` (default) or ``UNDERSCORE`` respectively.
 
+sub_symbol, sup_symbol
+  Define the chars that surround ``<sub>`` and ``<sup>`` text. Defaults to an
+  empty string, because this is non-standard behavior. Could be something like
+  ``~`` and ``^`` to result in ``~sub~`` and ``^sup^``.
+
 newline_style
   Defines the style of marking linebreaks (``<br>``) in markdown. The default
   value ``SPACES`` of this option will adopt the usual two spaces and a newline,

markdownify/__init__.py

@@ -66,14 +66,16 @@ def _todict(obj):
 
 class MarkdownConverter(object):
     class DefaultOptions:
-        strip = None
-        convert = None
         autolinks = True
+        bullets = '*+-'  # An iterable of bullet types.
+        convert = None
         default_title = False
         heading_style = UNDERLINED
-        bullets = '*+-'  # An iterable of bullet types.
-        strong_em_symbol = ASTERISK
         newline_style = SPACES
+        strip = None
+        strong_em_symbol = ASTERISK
+        sub_symbol = ''
+        sup_symbol = ''
 
     class Options(DefaultOptions):
         pass
@@ -325,6 +327,10 @@ class MarkdownConverter(object):
 
     convert_samp = convert_code
 
+    convert_sub = abstract_inline_conversion(lambda self: self.options['sub_symbol'])
+
+    convert_sup = abstract_inline_conversion(lambda self: self.options['sup_symbol'])
+
     def convert_table(self, el, text, convert_as_inline):
         return '\n\n' + text + '\n'
 

tests/test_conversions.py

@@ -201,3 +201,13 @@ def test_strong_em_symbol():
     assert md('<b>Hello</b>', strong_em_symbol=UNDERSCORE) == '__Hello__'
     assert md('<em>Hello</em>', strong_em_symbol=UNDERSCORE) == '_Hello_'
     assert md('<i>Hello</i>', strong_em_symbol=UNDERSCORE) == '_Hello_'
+
+
+def test_sub():
+    assert md('<sub>foo</sub>') == 'foo'
+    assert md('<sub>foo</sub>', sub_symbol='~') == '~foo~'
+
+
+def test_sup():
+    assert md('<sup>foo</sup>') == 'foo'
+    assert md('<sup>foo</sup>', sup_symbol='^') == '^foo^'