Fix output when input contains non-ascii character


main() returns a unicode object, and print has to encode this for output.  When output to terminal this usually works (because your terminal is unicode), but when output is to a file print defaults to ascii.

When I have a source file with a £ in it, I would previously get:
  File "plugins/jekyll-rst/rst2html.py", line 39, in <module>
    print(main())
UnicodeEncodeError: 'ascii' codec can't encode character u'\xa3' in position 1246: ordinal not in range(128)

With this change it works cleanly, albeit hard coded for utf-8.