More accurate transaction fee calculation

[lukechampine]

wallet.FundTransaction currently estimates the transaction size as len(inputs) * 241, which is inaccurate for transactions with few inputs and many outputs. I see two potential solutions: either take the number of outputs as an additional argument, or take an entire partially-filled transaction as an argument. I'm leaning towards the latter case, since it generalizes to things like file contract transactions and siafund transactions. The only downside is that passing in a partially-filled transaction is sort of a weird API.