gh_commit: Add get command **WIP**

[jcfr]

Todo:

• By default list all properties as:

  githubrelease commit REPO_NAME get SHA
  

  could output:

  sha 7921bb62e3b3240226acf66a57535219b9c3abac
  url https://api.github.com/repos/NeurodataWithoutBorders/pynwb/git/commits/7921bb62e3b3240226acf66a57535219b9c3abac
  html_url https://github.com/NeurodataWithoutBorders/pynwb/commit/7921bb62e3b3240226acf66a57535219b9c3abac
  author.name John Doe
  [...]
  

• support getting a single property using

  githubrelease commit REPO_NAME get SHA author.name
  

• support getting json output using

  githubrelease commit REPO_NAME get SHA --json
  

[ci skip]