Make gifs collaspible by default

[tcrawford-figure]

Addresses: https://github.com/N1ck/gifs-for-github/issues/62

Example output:

<details open>
  <summary><i>do it<i/></summary>
  <img src="https://media4.giphy.com/media/3o84sw9CmwYpAnRRni/giphy.gif"/>
</details>

Rendered output:

do it

[jmeridth]

@N1ck gentle bump. wdyt?

[N1ck]

@N1ck gentle bump. wdyt?

I've heard several opinions on how we might change or improve the way GIFs are output. I appreciate the simplicity of the extension as it currently stands, and I wouldn't want to make this the default behavior. However, perhaps we could introduce a way to configure the GIF output through a setting.

My initial thought is that you could set a 'template' for how the GIF is rendered, such as:

<details open>
  <summary><i>do it<i/></summary>
  {GIF}
</details>

The extension could then use this template, replacing {GIF} with the actual GIF.

This is not something I currently have time to implement but would be open to PR's!