Implement check for CHANGELOG

Closes #17, adds scanning a project for a CHANGELOG:

```
❯ flint --github pengwynn/rdio-cli
[WARNING] CHANGELOG not found
[INFO] Add a changelog to show what's changed with each release. http://keepachangelog.com/
```

/cc @technicalpickles who asked for it
/cc @olivierlacan who created the perfect link for this.