feat: add image options

[renbaoshuo]

This pull request resolves #140, closes #141.

Usage

_posts/foo.md

![](bar.svg)

<!-- Also works: -->

![](foo/bar.svg)

will be rendered to:

<p><img src="/post/foo/bar.svg"></p>
<!-- Also works: -->
<p><img src="/post/foo/bar.svg"></p>

Note that for some local markdown editor previews, we need to use ![](foo/bar.svg) to display the image correctly. Adding support for this insertion format can bring users a good local writing experience.

[SukkaW]

CI fails on windows. Do you have any clues?

[renbaoshuo]

CI fails on windows. Do you have any clues?

Nope.

[renbaoshuo]

CI fails on windows. Do you have any clues?

@SukkaW Fixed in 43dc603.