Make fully localizable via config

[freaktechnik]

Currently only Column names and labels can be localized. Other strings should also be localizable, most via config, though errors should arguably not require per-config localization.

In Config

• labels
• columns
• issue format (titles, placeholder text)

Others

• Tweet success
• Validation errors
• Reminders
• Tweet handling error reporting
• Default README? (Low prio, since like the issue template you can actually change the contents without affecting the tool)