You can hold all ERC-20 tokens in your wallet. These are tokens built on the Ethereum network.
You can find the full list at: https://etherscan.io/tokens
Why are some tokens excluded?
Argent's mission is to help people access a new web where you have more control of your data and identity. To do this we help people use Dapps (Decentralised apps). We believe the most exciting Dapps are being built on Ethereum, so we've focused on that to start.
Building on Ethereum also allows us to design the wallet so that it's safe and simple to use.
Do you support Bitcoin?
No, we do not, for the above reasons: we're focused on Dapps and having a wallet that's easy and safe to use.