add interlinks plugin for markdown

### Manage frequently used links in config file.

**Config example:**
```
interlinks.example=http://www.example.de
```

**Usage in markdown:**
```
[Normal boring link](http://www.example.com). 
But this is a [cool link](example>) that links to this site.
```

**generates:**
```
<a href="http://www.example.com">Normal boring link</a>. 
But this is a <a href="http://www.example.de">cool link</a> that links to this site.
```

Inspired bei interlinks plugin from pelican website generator.