Add new branch protection values

[tjamet]

GitHub has been adding some new fields in the branch protections
as documented:
https://developer.github.com/v3/repos/branches/#get-branch-protection

the documentation references the new fields in the branch protection as:

https://developer.github.com/v3/repos/branches/#get-branch-protection:
GET /repos/:owner/:repo/branches/:branch/protection

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

https://developer.github.com/v3/repos/branches/#update-branch-protection:
PUT /repos/:owner/:repo/branches/:branch/protection

field	type	required?
required_linear_history	boolean	optional
allow_force_pushes	boolean	optional
allow_deletions	boolean	optional

This PR achieves a similar goal as #1354, addressing comments.
Unlike suggestion in #1354 comment, the EnabledType is not defined as is, instead individual types are declared to prevent interface change if ever the field type changes.

Fixes #1345

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[tjamet]

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[gmlewis]

Thank you, @tjamet.
This is a good start, but please address the comments below and we can then proceed with the PR.

[gmlewis]

omitempty is spelled incorrectly each time. Please fix all three.

[gmlewis]

Every new comment in this PR needs to be a complete sentence ending in a period (".") so that the auto-generated Godocs will read nicely, and to be consistent with the rest of the repo. Please check each comment and fix as needed.

[gmlewis]

Although these three new struct names are good choices on their own, let's please rename them to match the name of the field, for two reasons... first, to be more consistent with the rest of the repo, and second to reduce confusion when searching for the struct or the field. So to be clear, let's please rename these structs to RequireLinearHistory, AllowForcePushes, and AllowDeletions, respectively.

[gmlewis]

Fixes #1345.
Closes #1354.

[tjamet]

Thanks for the fast feedback, request changes addressed

[gmlewis]

Thank you, @tjamet. LGTM. Merging.

(In the future, please try to remember not to force push commits to PRs for this repo, as it makes it more difficult for reviewers to see what changed since the last code review... not a big deal in this case, but if you make bigger PRs in the future, it is tremendously appreciated. Thanks. 😄)