Add slippage tolerance setting

[john-light]

Uniswap has a slippage tolerance setting that will cause a swap to fail if the trade results in slippage greater than x% of the price quoted to the user at the time they sign and broadcast the trade transaction. The user will lose the gas from their transaction but they at least won't end up overpaying by an unexpected amount.

Could convert.aragon.org implement something similar to prevent issues such as #41?

[sohkai]

Unfortunately not, to my knowledge. The bonding curve mechanism does not currently contain a slippage protection mechanism.