Add new challenge: Advanced Streaming

[swellander]

The original idea for the Advanced Streaming challenge was for it to be an evolution of the Eth Streaming challenge, adding the ability for partial withdrawals and streaming with any ERC20 token. However, it looks like the Eth Streaming challenge already includes partial withdrawals, making the ERC20 streaming feature the only difference between the level 1 and level 2 challenges.

@escottalexander should we add some more features to spice up the level 2? Or do you think the ERC20 streaming is enough of difference?

If ERC20 streaming is enough, I have a working version of the challenge, and just need a custom-stream branch created to PR into.

If you think the second challenge needs a bit more, some ideas to beef it up could be:

• the owner could choose a different token for each stream they create
• the user could choose which token they want to receive when withdrawing, maybe from an owner-whitelisted selection. And the contract would need to have a sufficient balance of the chosen token.
• the user could choose which token they want to receive when withdrawing, and the stream contract interacts with a uniswap router to swap ETH => desired token, then sends to the user

Let me know what you think. Thanks!

[escottalexander]

Awesome work! The custom-stream branch has been created. Feel free to submit a PR and we will go from there. Thanks!