Optionally override build comand or env vars per distributions

[bravomartin]

Lets say you want to be able to easily assign a different api target depending on the distribution.
on the options I would like to be able to either pass flags to the build command, or define env variables, or just be able to run a different command than the default.

...
"buildCommand": "npm run build",
	"environments": {
		"staging": {
			"stack": "my-app-staging",
			"bucket": "my-app-staging",
			"htmlCache": "60",
			"assetsCache": "31536000",
		"production": {
			"stack": "my-app-production",
			"bucket": "my-app-production",
			"htmlCache": "60",
			"assetsCache": "31536000",
>>                       "buildCommand": "npm run build:production"
		},
...