Add XacroFileContents substitution class

[bergercookie]

Add a substitution class for loading a xacro file during evaluation.

P.S. Midway through writing this I discovered that I could use the Command substitution to call to the xacro executable (#366). However I still believe there's value in having a dedicated class for in that

• it should be faster to call a pythn function than to invoke subprocess and
• it should be more straightforward for the developer at hand to discover and also use this substitution type for manipulating xacro files instead of concatenating the individual command parts

[clalancette]

Note that in order to do this, you would have to make xacro a dependency of this package, and that would mean pulling it into the ROS 2 core. Overall, I don't think this is a good move; while I know xacro is popular, it isn't the only way to do things. So I don't think it should be "special" and I think use of the Command substitution could just be better documented.

[bergercookie]

@clalancette, ok, thanks for the feedback, closing this.