Support markdown strikethrough

[thibaultamartin]

When someone uses the <del> and </del> tags in a message on Matrix, the same tag appear in the render.md file.

Hebbot should replace both of those tags with ~~.

Example:

• Matrix message: Version <del>1.1.0</del> 2.0.0
• Rendered markdown by hebbot: Version ~~1.1.0~~ 2.0.0

I think that can be achieved fairly simply by applying the following regex on the message:

• Match on <del>([^<\/del>]*)<\/del>
• Replace with ~~$1~~