Add hosted GitHub support

[JakeWharton]

For enterprise deployments

[JakeWharton]

one config proposal:

version = 0

[[github]]
host = "github.com" # default
user = "JakeWharton"
token = "123"

  [github.clone]
  watched = true

[[github]]
host = "github.example.com"
user = "jw"
token = "456"

  [github.clone]
  starred = true
  watched = true

alternatively, we could support multiple .toml configs in the directory, but then you almost want a different model for each toml (git, github, gitlab, etc.) so you'd have to use a naming scheme or directory structure which complicates the simple case

[JakeWharton]

a bonus to the one-config approach, if you have multiple user accounts, for whatever reason, this would also help you