Number of tildes for strikethourough should be two

[0x6b]

Hi @domchristie thank you very much for creating such a nice code. I used your turndown with my firefox add-on, copy-selection-as-markdown, and it works very well. Recently I found this plugin and tried to integrate with my add-on, then found a difference between your implementation and GFM spec.

According to GitHub Flavored Markdown Spec, strikethrough text should be wrapped in two tildes.

6.5 Strikethrough (extension)

GFM enables the strikethrough extension, where an additional emphasis type is available.

Strikethrough text is any text wrapped in two tildes (~).

But your implementation only adds one tilde as follows:

https://github.com/domchristie/turndown-plugin-gfm/blob/61a981b8c6aaec73bbb8a844d9f8686d0d5f066e/src/strikethrough.js#L5

Could you please have a look into the spec and give me your thought?