Use the IMGD_LISTENON env if present

Add support for setting a different listen address at runtime.
Add IMGD_LISTENON env to README

Using a generic named environment variable. Matching README to feature.
https://minotar.net/ redirect requires a 302
Merge pull request #90 from LukeHandle/patch-2

https://minotar.net/ redirect requires a 302
Merge pull request #88 from LukeHandle/patch-1

Listen on specific address/port if Env is set
Merge remote-tracking branch 'upstream/master'