Add git config user.email and user.name

[sorenlouv]

Closes #270

This will infer the git config values user.email and user.name from the source repository. This is necessary if no global git config values are set.

It is also possible to set the username and email manually in the .backportrc.json config:

{
    "accessToken": "***",
    "gitUserName": "Your Name",
    "gitUserEmail": "you@example.com"
}