Support for `[!Important]`

[trusktr]

GitHub's markdown now supports [!Note], [!Important], and [!Warning], but not [!Tip] or [!Attention].

Currently docsify-plugin-flexible-alerts is missing [!Important]. Would be nice to get that one, to maximize compatibility with GitHub.

Great plugin btw!

[fzankl]

GitHub supports [!NOTE], [!TIP], [!IMPORTANT], [!WARNING] and [!CAUTION].

https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts

I will modify the plugin to support further alert types.