Support multiple upstreams

[fredbi]

• Upstream per resource (global upstream as default)
• Upstream (global or resource) may be an URL with path: will be prepended to request path
• Upstream (resource) may specify the prefix to strip on incoming request

Ex:

• resource URL: /api/path

• upstream: https://localhost/prepend

• stripped: /api

• incoming: https://api/path/resource

• routed to: https://localhost/prepend/path/resource

Signed-off-by: Frederic BIDON frederic@oneconcern.com WIP:

• [ ] integration tests

Fixes #14