Disclaimer: I work for Atlassian.

Yes of course they'll support github too. Check out the facts in here: http://www.sourcetreeapp.com/acquisition-faq/

Here's the quote: Will SourceTree drop support for non-Atlassian products (e.g. GitHub)?

Absolutely not! Atlassian is completely committed to empowering developers to use whatever tools they choose; artificially limiting choice is simply not on the agenda.