Globbing for <xacro:include>

In addition to:

~~~
<xacro:include filename="$(find my_package)/other_file.xacro" />
~~~

There would be support for:

~~~
<xacro:include filename="/path/to/my/extras/*.xacro" />
<xacro:include filename="/path/to/my/extras/*" />
~~~

My interest is in using xacro to build up `launch` and `urdf`s at launch-time from installed packages, based on a platform configuration that is not known when the packages are built. So the launch and urdf are constructed using globs to mix-in additional components that correspond to accessories configured on our vehicles.

At present, we achieve the desired effect using [mklaunch](https://github.com/clearpathrobotics/robot_upstart/blob/hydro-devel/scripts/mklaunch) and [mkxacro](https://github.com/clearpathrobotics/robot_upstart/blob/hydro-devel/scripts/mkxacro) scripts in the robot_upstart package, but I believe this functionality would be best provided by xacro—would be happy to prepare a PR for indigo.

Thoughts?