go/client: cmd line auto-detect pre-cert upload
go/client: cmd line auto-detect pre-cert upload

Tested by retrieving a pre-cert chain from Aviator with
getentries, then re-upload to Pilot with upload and confirm
that the SCT signature worked.

RJPercival/certificate-transparency,pphaneuf/certificate-transparency,grandamp/certificate-transparency,RJPercival/certificate-transparency,grandamp/certificate-transparency,lexibrent/certificate-transparency,grandamp/certificate-transparency,pphaneuf/certificate-transparency,RJPercival/certificate-transparency,phad/certificate-transparency,RJPercival/certificate-transparency,google/certificate-transparency,phad/certificate-transparency,phad/certificate-transparency,phad/certificate-transparency,pphaneuf/certificate-transparency,grandamp/certificate-transparency,pphaneuf/certificate-transparency,google/certificate-transparency,google/certificate-transparency,lexibrent/certificate-transparency,google/certificate-transparency,lexibrent/certificate-transparency,lexibrent/certificate-transparency