Option to write render_html or render_latex output to a file

This is especially helpful for render_latex, since a common workflow is to store each tex component as separate files, and then include them in the master tex file. It'd be nice to have a `render_latex(out='out.tex')` option.

Current workaround (which would probably be the way to implement the feature) is:
```
f = open("out.tex", "w")
f.write(stargazer.render_latex())
f.close()
```