Release md v0.3.1

[lempiji]

I have created a tool that executes the code blocks included in 
Markdown.

http://github.com/lempiji/md

I'm sure some of everyone have experienced that the code examples 
in the documentation don't work.

With this tool, you can execute Markdown code directly and always 
validate it with CI.

```
dub run md -- README.md
```

The syntax of dlang is not affected by indentation, and `import` 
can be written anywhere, so it was a very simple mechanism to 
achieve this.

However, there may be a better way. If you found it, please try 
to create it.

I think dlang ❤ Markdown