feat: disable colored output if NO_COLOR is defined

[buchdag]

This is a rebase of ddollar#124 by @sehrope

This PR is a rebased version of #122 with a couple more commits added by me. That original PR improves the error handling and adds automatic disabling of color output based on whether stdout is a terminal.

This PR expands on that with a few more commits to:

1. Checks for a NO_COLOR env variable and, if defined, disables color output regardless of whether we're running in a terminal (per https://no-color.org/)

$ NO_COLOR=1 forego start
...

2. Optimize things a bit by only creating the "left" outlet formatter once. Previously it was being generated for ever line of output but the format itself never changes so it's wasted cycles.

The original PR also included this:

1. Add injecting a VERSION Makefile variable so you can include the date / commit in the resulting build. Defaults to current value of "dev":

$ make VERSION="$(date --utc +%Y%m%d-%H%M%S)-$(git show-ref -s -- HEAD)" build
...
$ forego version
20200605-120321-0bdbf9b27cf9da13e246ff1ba90ab5961f4be849

Which has been replaced by a similar version injection closer to what we already do in docker-gen.