Allow change source for MathJax

The original MathJax source is not fast in China, maybe provide an option to replace it wound be help, like the following code:

``` python
if key == 'source':
    try:
        typeVal = isinstance(value, basestring)
    except NameError:
        typeVal = isinstance(value, str)

    if not typeVal:
        continue

    mathjax_settings[key] = "'".join(["", value.strip('\'"'), ""])
```
