Crowdsourcing: allowing others to add funding to a bounty

**User Story:**
Start a bounty and have others add to the funding as well.

For example, I created the following bounty on Bountysource, I seeded it with $100. I then used my social network to campaign for others to contribute to it. As you can see, we are up to $2860.

In short, I'd like to do the same with Gitcoin.

https://app.bountysource.com/issues/95048565-make-construction-drawing-generation-like-really-really-awesome