add cacheDir flag (#11)

golang version of #7 

-----
This enables setting a custom cache directory for Hugo. Some caching
plugins (e.g. http://plugins.drone.io/drillster/drone-volume-cache/)
require the directories to be cached to be located inside the workspace.

cmd/drone-hugo/main.go

@@ -36,6 +36,12 @@ func main() {
 			Usage:  "include content with publishdate in the future",
 			EnvVar: "PLUGIN_BUILDFUTURE",
 		},
+		cli.StringFlag{
+			Name:   "cacheDir",
+			Usage:  "change cache directory (useful when using caching plugins)",
+			EnvVar: "PLUGIN_CACHEDIR",
+			Value:  "",
+		},
 		cli.StringFlag{
 			Name:   "config",
 			Usage:  "config file (default is path/config.yaml|json|toml)",