Expand call_trace

[iamdefinitelyahuman]

What I did

Expand TransactionReceipt.call_trace to include inputs and return values for external calls.

Closes #625

How I did it

• Add the TransactionReceipt.subcalls attribute. This is a trace property that records all the subcall info while the trace is expanded. See the updated documentation for more info on how it is structured.
• Add the _ContractMethod.decode_input method for decoding input arguments.
• Refactor the tree-generation logic into brownie.utils.output.build_tree. The new approach uses nested dicts and lists. It's far more powerful and better documented than the previous approach. For now I haven't touched other tree diagrams, but in the v2 refactor everything will be refactored to use this method.
• black is now a proper brownie dependency, rather than a dev-dependency. I'm using it to handle formatting for inputs and return values, so there isn't any weirdness when displaying lengthy arrays. For v2 i think some of the home-rolled formatting code in the console can be replaced with black.