Add button on GitHub's source code page

[zenorocha]

[Naramsim]

Since we don't actually have a plain code here, we could fetch the file from Rawgit API and copy it to the user clipboard.

Like: https://rawgit.com/zenorocha/codecopy/master/src/styles/main.scss

[FranklinYu]

RawGit is dead. GitHub actually provide the raw file link itself, no API needed: https://github.com/zenorocha/codecopy/raw/master/src/styles/main.scss. This has the benefit that the extension doesn’t require new domain permissions.