Pytest stdout capturing

[iamdefinitelyahuman]

What I did

1. Allow -s flag in brownie test
2. Add PytestPrinter class to improve readability of output when stdout capture is disabled.

Closes #389

How I did it

1. When stdout capture is disabled, verbose output is automatically enabled.
2. PytestPrinter replaces the builtin print method during test execution to:
   a. add a newline before the first line
   b. add a second test id prior to reporting the test result

The final output ends up looking something like this:

tests/test_transfer.py::test_transfer RUNNING
this is some text from a call to print
tests/test_transfer.py::test_transfer PASSED

How to verify it

Run tests.