Cannot configure status labels

[tetsuharuohzeki]

Some projects or organizations have already used labels which are similar to S-awaiting-merge, S-awaiting-review, or others. If we introduce this bot to such projects, we need to configure the source code.

Of course, an operation bot should be customized heavily for each projects, it's not wrong way that user forks this source code.

However, its way also makes it hard to upstream their patches which introduces new general useful features. So I think it's better to make labels configurable.