Dependent logs

Adds dependent changelog injection, as requested at #21 

Dependent changelog references are picked up by the guthub issue matching this regex:
```
(update|upgrade) (\w+)\/(\w+) to (.*) from (.*)
```
The reference must be to the github path, such as:
```
update creativelive/mdchangelog to 0.4.0 from 0.3.0
```
The semver entries have to match milestone titles in the reference repo.