Allow setting the filename using ROS parameter in image_publisher node (

#835)

Current implementation of `image_publisher` node doesn’t allow the user
to set the published image file name using the ROS parameter
(`~filename`). It just exits with the `image_publisher requires
filename` error.

But using parameter is very natural for ROS, and it's useful as well:
you can set filename in a launch file using `command` attribute of
`param` tag (and other features of parameters). Like this:

```xml
<node pkg="image_publisher" type="image_publisher" name="image_publisher">
    <param name="filename" command="<call some script to determine file name>"/>
</node>
```

For my project, I need a more complex logic to determine the name of the
published file, and I was surprised this doesn't work (non-obvious from
the documentation).

image_publisher/src/node/image_publisher.cpp

@@ -38,13 +38,16 @@ int main(int argc, char **argv)
 {
  ros::init(argc, argv, "image_publisher", ros::init_options::AnonymousName);
 
- if (argc <= 1) {
- ROS_ERROR("image_publisher requires filename. Typical command-line usage:\n"
- "\t$ rosrun image_publisher image_publisher <filename>");
- return 1;
+ if (!ros::param::has("~filename")) {
+ if (argc <= 1) {
+ ROS_ERROR("image_publisher requires filename. Typical command-line usage:\n"
+ "\t$ rosrun image_publisher image_publisher <filename>");
+ return 1;
+ }
+
+ ros::param::set("~filename", argv[1]);
  }
 
- ros::param::set("~filename", argv[1]);
  nodelet::Loader manager(false);
  nodelet::M_string remappings;
  nodelet::V_string my_argv(argv + 1, argv + argc);