Errorx Versions Save

• Fixed panic on unwrapping nil typed as *errorx.Error https://github.com/joomcode/errorx/pull/42

Added support of go 1.13 errors package:

• errors.Unwrap() works properly for errorx.Error
• errors.Unwrap() respects opaque (errorx.Error::Wrap()) and transparent (errorx.Decorate()) wrapping contract
• Unwrapping via errors.Unwrap() works with chains of errorx and non-errorx errors
• The same behaviour is true for errors.Is()

Add go modules support.

• Panic() now handles stack traces better, especially across multiple goroutines