mount helper: add support for a key file option.

This is pull request #182 from vaygr/key_file-fstab.

docs/mounting.md

@@ -22,6 +22,10 @@ Console:
 
     GOOGLE_APPLICATION_CREDENTIALS=/path/to/key.json gcsfuse [...]
 
+When mounting with an fstab entry, you can use the `key_file` option. For example:
+
+    my-bucket /mount/point gcsfuse rw,noauto,user,key_file=/path/to/key.json
+
 [gce]: https://cloud.google.com/compute/
 [gce-service-accounts]: https://cloud.google.com/compute/docs/authentication
 [gcloud tool]: https://cloud.google.com/sdk/gcloud/
@@ -97,6 +101,7 @@ with dashes instead of underscores:
 *   `implicit_dirs`
 *   `dir_mode`
 *   `file_mode`
+*   `key_file`
 
 On both OS X and Linux, you can also add entries to your `/etc/fstab` file like
 the following:

tools/mount_gcsfuse/main.go

@@ -86,6 +86,14 @@ func makeGcsfuseArgs(
 				"--"+strings.Replace(name, "_", "-", -1),
 			)
 
+		// Special case: support mount-like formatting for gcsfuse string flags.
+		case "key_file":
+			args = append(
+				args,
+				"--"+strings.Replace(name, "_", "-", -1),
+				value,
+			)
+
 		// Special case: support mount-like formatting for gcsfuse string flags.
 		case "dir_mode", "file_mode":
 			args = append(