Handle unicode characters when reading from files

I have a blog post containing the Danish letter 'ø', and when I render the site I get the error message:

    UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 453: ordinal not in range(128)

This pull request changes the `open()` call to specify UTF-8 as the encoding if reading from a file which isn't opened in binary mode, and that solves the problem.