Update dependency to include RFC 8555 impementation of acme protocol

Rename file to main.go

Enable support for ACME http-01 challenges.

This is now required since ACME Letsencrypt disabled tls-sni challenges:
https://letsencrypt.status.io/pages/55957a99e800baa4470002da

Add "X-Forwarded-Proto: https" header

Some backends may depend on this to properly assemble urls.

Depend on newer autocert package

Configurable read/write timeouts

Fix backend misusage

Fix a bug that could lead to using only the last backend for all
hostnames due to variable redefining.

See https://play.golang.org/p/yjiEENusQs as an illustration

Add -hsts flag

Add support for static sites handling