Added support for a custom settings.xml.

I went with the simplest approach for getting a custom `settings.xml` into the slug. I did not include http://s3.amazonaws.com/heroku-jvm-langpack-java/settings.xml as it did not seem necessary. 

I also did not include `settings-security.xml`. Instead, users can create config vars with secrets such as passwords or URLs. These can then be used in the settings.xml as `${env.MVN_PRIVATE_REPO_PASSWORD}`.

@jsimone used this as a fallback in custom_settings branch:
```
curl --silent --max-time 10 --location $MAVEN_SETTINGS_URL --output .m2/settings.xml
```
Do you think that would better, or just just an env var that expects a file a in the build dir?