Add ARM builds

[benjaminoakes]

Thanks for gron!

I've found myself wanting it on a Raspberry Pi and potentially other ARM devices (I'm typing this on an ARM Chromebook with Crostini, case in point). I'd love to see prebuilt binaries for armv7l and any other popular ARM architectures. I'm betting macOS on ARM will make this a popular request.

[benjaminoakes]

I put this together with roughly the following, running on a Raspberry Pi 3:

host$ docker pull golang
host$ mkdir -p "$HOME/out"
host$ docker run -v "$HOME/out:/go/out" -it --rm golang sh
container# go get -u github.com/tomnomnom/gron # I assume this is HEAD of master
container# cp bin/gron out
host$ cd out
host$ zip gron-linux-armv7l-602235e.tgz gron # 602235e is HEAD of master when I built this

(Just a summary of my shell history, though I think it's correct.)

Output attached for others' benefit. 😃

gron-linux-armv7l-602235e.zip