Specifying both --html and --fragment outputs just the fragment

Consider:
```
nabijaczleweli@tarta:~/code/klapki/man$ ls
index.txt  klapki.md
nabijaczleweli@tarta:~/code/klapki/man$ ronn -f5 klapki.md
html_fragment: ./klapki.8.html_fragment
nabijaczleweli@tarta:~/code/klapki/man$ ls
index.txt  klapki.8.html_fragment  klapki.md
```