Currate stdout and stderr in "core" #849

As noted a comment in #828, the "core" of git-bug should support the CLI's need for parsable text output. For the purposes of this issue, "core" comprises the following packages: cache, entities and repository. 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.
• During "core" initialization, stderr should be redirectable.
• All error conditions should be returned to callers via Go errors without output to either stdout or stderr.