Allow <!-- CODE --> and <!-- PROP_STABLE --> snippets (React)

[ChristiaanScheermeijer]

First of all, thank you for this addon!

Before I'd installed this addon, I was playing with @storybook/addon-info. I really liked the autogenerated code and props table snippet from this addon.

So I was thinking, it would be really cool if we could do something like this in the README.md:

# Avatar

### Usage

This code is autogenerated based on the code in the `storiesOff#add` function:

<!-- CODE -->

### Preview

<!-- STORY -->

### Properties

A Markdown table with all props generated from the defaultProps and propTypes:

<!-- PROPS_TABLE -->

[tuchk4]

@ChristiaanScheermeijer Nice idea!
I will definitely do it in next release. Planning to do it at the start of next week

[tuchk4]

Will be released with #118

[tuchk4]

@ChristiaanScheermeijer released at storybook-readme@5.0.0-alpha.150

Here is docs

Here is Story
<!-- STORY -->

Here is Props table
<!-- PROPS -->

Here is docs after props table

Note that right now there are breaking changes at alpha version. I wrote about here #118 (comment)

[tuchk4]

feature added at 5.0.0

[deini]

@tuchk4 Are there any plans to support <!-- CODE --> so that we can handle the use case the description of this issue mentions?