* define BUNDLE_NAME after including build/setup.mk (just for clarity, not correctness)
* separate plugin id from version with dash in bundle name to match Mattermost package
* fix various js/go idioms (some pre-existing!)
* fix error message
* warn upfront if go/dep/npm not available
* try to find dep in $GOPATH/bin if not installed globally
* only override variables if not externally defined, allowing manual override from command line
* split up Makefile into a buid/setup.mk for simplicity
I didn't actually find anything that made this Makefile not-linux
specific -- tested on Debian with the standard make installed.