Add ability to specify non-standard location for kubeconfig

[johnharris85]

The code that pulls in the kubeconfig here doesn't seem to allow any other location than /home/<user>/.kube/config (unless I'm reading / testing it incorrectly). Would be good to pull from the KUBECONFIG env var here maybe (respecting kubectl order of precedence)?

Hit this because my config in /home/<user>/.kube wasn't valid, but my KUBECONFIG was set correctly, and I got kind of an esoteric error:

F0311 14:25:12.071505   19915 mp-compiler.go:91] invalid configuration: no configuration has been provided
goroutine 1 [running]:
github.com/golang/glog.stacks(0xc4204e8000, 0xc42019a000, 0x6b, 0xb9)
	/home/bburns/gopath/src/github.com/golang/glog/glog.go:769 +0xcf
github.com/golang/glog.(*loggingT).output(0x197d940, 0xc400000003, 0xc4200ba8f0, 0x18d9175, 0xe, 0x5b, 0x0)
	/home/bburns/gopath/src/github.com/golang/glog/glog.go:720 +0x345
github.com/golang/glog.(*loggingT).printf(0x197d940, 0x3, 0xc4202d3600, 0x39, 0x0, 0x0, 0x0)
	/home/bburns/gopath/src/github.com/golang/glog/glog.go:655 +0x14c
github.com/golang/glog.Fatalf(0xc4202d3600, 0x39, 0x0, 0x0, 0x0)
	/home/bburns/gopath/src/github.com/golang/glog/glog.go:1148 +0x67
main.main()
	/home/bburns/gopath/src/github.com/metaparticle-io/metaparticle-ast/cmd/compiler/mp-compiler.go:91 +0x612

Happy to submit a PR if you're happy with the solution (look at KUBECONFIG first).