Enable toc-title in default.html

[atusy]

Recent pandoc has an option toc-title.
This PR updates default.html to enable the option.
It works bot for toc_float as well.

For the consistency with other parameters, maybe it is a choice to add toc_title as a parameter of html_document.
If so, we should also consider adding the parameter to other output formats (e.g., word_document)

---
output:
  html_document:
    toc: true
    toc_float: true
toc-title: TOC
---

# foo

# bar

[yihui]

Thank you!