#75 add ExtOrigin config to handle lfs-test-server behind a reverse proxy case

[leyuanpanbluvec]

As described in #75, when the lfs-test-server is behind a reverse proxy, the LFS_HOST will not denote the external link correctly. For example, the external link is https://git-lfs.example.com served by nginx, the LFS_LISTEN is tcp://localhost:1234 and LFS_SCHEME is http. Whatever LFS_HOST is set, the external URL is incorrect.
I added additional config, i.e., LFS_EXTORIGIN, which is blank by default. If it is blank, the old method will be used to denote the external origin, as follows:

if Config.IsHTTPS() {
	Config.ExtOrigin = fmt.Sprintf("%s://%s", Config.Scheme, Config.Host)
} else {
	Config.ExtOrigin = fmt.Sprintf("http://%s", Config.Host)
}

However, when the user set LFS_EXTORIGIN, it will be used to skip the old setting, e.g., LFS_EXTORIGIN='https://git-lfs.example.com'.

By the way, why not use Config.ExtOrigin = fmt.Sprintf("%s://%s", Config.Scheme, Config.Host) to replace the above old external link generation method?

[chrisd8088]

Thanks for the PR! I made a couple of small comments only. If you want to add some additional commits to address those, that's great, and if not, I'll merge this as it stands.

[chrisd8088]

Looks great, thank you for the contribution!