Change default character encoding

[samihda]

I changed the default character encoding in the generated preview document to UTF-8.

No breaking changes were introduced, but I'm happy to adjust this in case we're okay with dropping markdown-content-type altogether and switching to the charset attribute instead.

Related Issue

I came across this when text doesn't render properly in the preview. It was German text in my case, but I'm sure it affects others too. This should also fix #340.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING.md document.
• I have updated the documentation in the README.md file if necessary.
• I have added an entry to CHANGES.md.
• I have added tests to cover my changes.
• All new and existing tests passed (using make test).

[jrblevin]

Thanks for the patch!