Update gfm.cson for Kotlin highlighting

[radixdev]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

Adds Kotlin syntax highlighting for code blocks.

Alternate Designs

n/a

Benefits

The Kotlin language can now have syntax highlighting when editing markdown code blocks. This increases readability when editing markdown.

Github supports Kotlin highlighting, see the below.

    // Firebase tokens cannot be obtained on the main thread.
    Thread(Runnable {
      try {
        val token = FirebaseInstanceId.getInstance().getToken("<YOUR_SENDER_ID>", "FCM")
        Appboy.getInstance(applicationContext).registerAppboyPushMessages(token)
      } catch (e: Exception) {
        Log.e(TAG, "Exception while registering Firebase token with Braze.", e)
      }
    }).start()

Possible Drawbacks

I do not foresee any drawbacks to Kotlin language highlighting.

Applicable Issues

https://github.com/atom/language-gfm/issues/166

[rsese]

Sorry about that @radixdev - just wanted to acknowledge that someone from the team will take a look as soon as they can.

[winstliu]

Thank you!