Add Telegram icons to support Jetpack Telegram share

[JonahBraun]

Icons for Telegram supporting Automattic/jetpack#3775 (itself a completion of Automattic/jetpack#3226).

Please hold on this PR until Automattic/jetpack#3775 is approved. There might be further changes, or cancelled altogether in favor of a different approach (e.g. SVG, see technical notes in other PR) in light of #54.

Two icons are added, both based on on Telegram's "flat" icon (suitable for monochrome).

[jasmussen]

Hey thank you for making a pull request like this. Much appreciated! Despite the ideology of Genericons to add "no more social icons" when there's a big green merge button like this it's really tempting to just press it.

However, lately we've been doing some rethinking of the future of Genericons. That rethinking is specifically happening here: https://github.com/Automattic/Genericons/tree/refactor/genericons

The long and short of it is that we'd like for the next version of Genericons to move to a new build process, and shed a bunch of icons to focus things on icons only, no logos. Thankfully, there's a great new set we can point to, which will accept virtually any logo you can throw at it: https://github.com/Automattic/social-logos

I believe Telegram is already part of that set.

[JonahBraun]

Awesome, thanks for pointing me to Automattic/social-logos @jasmussen. The next PR I was going to open was to fix some of the other social icons, particularly G+ in the current Genericons is terrible. Automattic/social-logos looks like it should address that!

Closing this PR.