Improving title formatting in YAML references

[akosztolanyi]

• Zotero uses html tags for title formatting
• creating a pdf output from a markdown document using YAML references looses formatting because pandoc replaces e.g. "..." with "<i>{=html}...</i>{=html}", and thus pdf output will ignore the formatting
• substituting html tags with markdown formatting solves the issue
• _sanitize_yaml function in zotero.py is extended to make this substitutions (similarly to the substitutions in _get_bib_ref)

[jalvesaq]

Thank you!