Add support for creating scoped installation tokens

[tarebyte]

See https://developer.github.com/v3/apps/#create-a-new-installation-token

This creates a new function CreateScopedInstallationToken, which allows the caller to limit an installation's access to a subset of repositories and permissions.

These parameters are optional which is why I opted to created a new function for this.

✨ Feedback appreciated ✨

[codecov]

Codecov Report

Merging #1182 into master will decrease coverage by 0.01%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master    #1182      +/-   ##
==========================================
- Coverage   70.25%   70.24%   -0.02%     
==========================================
  Files          84       84              
  Lines        5867     5878      +11     
==========================================
+ Hits         4122     4129       +7     
- Misses        956      958       +2     
- Partials      789      791       +2

Impacted Files	Coverage Δ
github/apps.go	65.3% <63.63%> (-0.22%)	⬇️

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[gmlewis]

Thank you, @tarebyte!

We try to keep the surface area of the API as small as possible (to make maintenance easier, to make it easier for the user to find a function they need, and to keep the auto-generated godocs as small as possible), and prefer to break the API and bump the major version number.

I would like to request that you add the new parameter to the existing endpoint above it, but before you make this non-trivial change, let's see if @gauntface agrees with me or if he prefers to keep it separate like you've made it.

Otherwise, LGTM.

[gmlewis]

friendly ping @gauntface

[gauntface]

I would agree with @gmlewis that moving to a single struct would make this more consistent with the rest of the API. Otherwise, this change is LGTM from me