Scan existing labels from a repository and save it to a file

[tonglil]

Create a reader package with a Run function that pulls all of the repos from a remote, converts the GitHub object into types.LabelFiel, and writes it into a file.

See: https://github.com/tonglil/labeler/blob/master/writer/writer.go#L84

Bonus functionality: merge with a file if the file already exists.

[tonglil]

Closed by #13.