Support multi-line backend extras with line separator ||

[chrsch]

This is a first step to provide a solution for #85.

If someone could point out how to bring on the BackendExtraMultilineSeparator option I can implement this as well.

[thomasjpfan]

Thanks for the PR!

Editing how the template process strings, would make the code harder to follow. I would prefer BackendExtra be edited when GetServiceFromProvider here:

docker-flow-proxy/proxy/types.go

Line 326 in acff954

func GetServiceFromProvider(provider ServiceParameterProvider) *Service {

In GetServiceFromProvider, the backendExtraMultilineSeparator can be queried. If it is defined and has a non-zero length, it can be used to separate BackendExtra into multiple lines.

• Test in types_test.go to make sure that BackendExtra is separated correctly.
• This new feature would need to be documented in usage.md.