pandoc 转换 md 到 pdf 时，出现下面的错误：
$ pandoc test.md -o test.pdf -t beamer --latex-engine=xelatex --template=beamer-template.tex
pandoc: Error producing PDF from TeX source.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! fontspec error: "font-not-found"
!
! The font "Arial" cannot be found.
!
! See the fontspec documentation for further information.
!
! For immediate help type H <return>.
!...............................................
解决方法：
ubuntu 下执行下面命令：
apt-get install ttf-mscorefonts-installer