fix link parsing

[zkamvar]

This fixes link parsing to take into account links that have markdown inside them.

[a _link_ with **markdown**]({{ page.root }}/linkie)

This will fix #120

This also updates jekyll image attribute parsing and takes into account attributes that are situated on a separate line as the image:

![image alt text](https://placekitten.com/200/200)
{.image-with-shadow}

This addresses #119, but does not necessarily fix it because figure attributes on new lines are not allowed (afaict):

pandoc::pandoc_convert(text = "![a](link)\n{alt='b'}", to = "html")
#> <p><img src="link" alt="a" /> {alt=‘b’}</p>
pandoc::pandoc_convert(text = "![a](link){alt='b'}", to = "html")
#> <figure>
#> <img src="link" alt="b" />
#> <figcaption>a</figcaption>
#> </figure>

^{Created on 2023-04-18 with reprex v2.0.2}