Add a range of Kana characters

[raimon49]

Hi, Thanks for the great plugin.

In the Japanese language, the characters are called "Kana" characters. For example, the famous character Hatsune Miku.

I hope this plugin also supports Kana characters.

Before:

# About Hatsune Miku

<!-- vim-markdown-toc GFM -->

* [初音ミクV3について](#初音v3)

<!-- vim-markdown-toc -->

## 初音ミクV3について

After:

# About Hatsune Miku

<!-- vim-markdown-toc GFM -->

* [初音ミクV3について](#初音ミクv3について)

<!-- vim-markdown-toc -->

## 初音ミクV3について

[mzlogin]

Thanks for your PR!