Xacrodoc

Additions
* There is now a `xacrodoc.packages.update_package_cache` function, which allows the user to directly pass a dictionary mapping package names to paths in order to resolve `$(find)` and other package look-ups. Furthermore, package paths that are resolved through the other look-up mechanisms are also cached, which should bring some performance benefits.

Changes
* This release incorporates https://github.com/ros/xacro/pull/339, which enables `$(find)` to be nested in an `$(arg)`. This behaviour was supported in the ROS1 versions of xacro, but had been lost through some refactoring, and was thus also not in xacrodoc.

xacrodoc is now fully functional whether or not ROS is installed on the system (previously, only a subset of xacro files could be compiled without ROS).

In particular, there is now a `packages` submodule that controls how xacrodoc resolves package names in xacro files. It uses ROS infrastructure when available, but can also be configured by the user. See the README for more details.

Thanks to stephane-caron for some helpful feedback!

First official release. Now we have [docs](https://xacrodoc.readthedocs.io/en/latest/)!