Make font configurable

When generate Chinese chars to pdf, I got this error:

```
$ wordsmith generate pdf
Generating pdf...
pandoc: Error producing PDF from TeX source.
! Package inputenc Error: Unicode char \u8:简 not set up for use with LaTeX.

See the inputenc package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.77 \section{简介}

Try running pandoc with --latex-engine=xelatex.
Error: pdf generator failed

```

Through http://johnmacfarlane.net/pandoc/faqs.html , I add a `font` configuration item to `.wordsmith`.

Please consider it.

Thanks,
Bin