Add support for email forwarding (re-sending)

[charyorde]

From here, http://elasticinbox.github.io/api/ , I couldn't see support for email forwarding.

[charyorde]

Since this is not supported OOTB, looking at the source code, I'm tinkering with:

• Write a custom filter(of com.elasticinbox.lmtp.filter.Filter) that would parse incoming email and modify the envelop recipient address. This would be based on the internal LMTP.

Any ideas will be much appreciated.