Currate `stdout` and `stderr` in "core"

[smoyer64]

As noted a comment in #828, the "core" of git-bug should support the CLI's need for parsable text output. In order to enable this behavior, the "core" should have the following traits:

• [ ] There should be no output to stdout
• [ ] Minimize the amount of output to stderr
• [ ] During "core" initialization, both stdout and stderr should be redirectable.
• [ ] All error conditions should be returned to callers via Go errors without output to either stdout or stderr.