Make bodywidth kwarg overridable using config

Default values for arguments are evaluated at import time.
This makes a problem when using html2text the following way:

    import html2text  # The default value is evaluated here
    html2text.config.BODY_WIDTH = 0
    text = html2text.html2text(html)

It is thus necessary to wait until the function runs to fetch the actual
configuration value.

html2text/__init__.py

@@ -819,7 +819,9 @@ def optwrap(self, text):
         return result
 
 
-def html2text(html, baseurl='', bodywidth=config.BODY_WIDTH):
+def html2text(html, baseurl='', bodywidth=None):
+    if bodywidth is None:
+        bodywidth = config.BODY_WIDTH
     h = HTML2Text(baseurl=baseurl, bodywidth=bodywidth)
 
     return h.handle(html)