add encoding for read md file

[Lchiffon]

read_file function should have a rgument for encoding="UTF-8"

Can't use Chinese character in Windows without that~

[GegznaV]

@Lchiffon your suggestion to add encoding="UTF-8" in read_file is really helpful. It enabled me to have possibility to use my native language in slidify slides. I hope your bug fixes and pull request will be accepted shortly.