Add support for new fields in Branch Protection

[pcantea]

Fixes #1345

[wesleimp]

Thank you, @pcantea
👌

Awaiting second LGTM before merging.

[gmlewis]

Sorry, @pcantea, but this PR will not work as written. See details below.

[gmlewis]

Each of these three bool fields need to be structures with a single Enabled field, based on the GitHub v3 API developer docs found here:
https://developer.github.com/v3/repos/branches/#get-branch-protection

Here is the example output from this endpoint:

...
  "required_linear_history": {
    "enabled": true
  },
  "allow_force_pushes": {
    "enabled": true
  },
  "allow_deletions": {
    "enabled": true
  }

Since each of these structs appear to only have a single Enabled field, I'm fine with making them all the same struct type... something like:

type EnabledType struct {
  Enabled bool `json:"enabled"`
}

and then these three fields would have struct pointers with omitempty, like this:

RequiredLinearHistory *EnabledType `json:"required_linear_history,omitempty"`
AllowForcePushes      *EnabledType `json:"allow_force_pushes,omitempty"`
AllowDeletions        *EnabledType `json:"allow_deletions,omitempty"`

That way, if GitHub provides the values, great... but if not, we know that they weren't returned.

[gmlewis]

Unfortunately, this won't work either. Because these are optional fields, these three need to be type *bool and not bool.
They also need the ,omiteempty flag on them

[wesleimp]

Sorry for my last review, I was a little bit confused about the docs. My bad