Option to load colors from XResources

[karlch]

As mentioned in #62 loading colors from xresources is a great feature.

After merging #123, the vimiv part of the implementation should be straightforward. It requires:

1. A function to replace an X resource string with the correct value similar to getenv in config.external_configparser.
2. Adding this function with a corresponding prefix to ExternalInterpolation.CONVERTERS.

In case another dependency is used, it should stay optional following the

try:
    import xresourcelib
except ImportError:
    xresourcelib = None

pattern and checking for if xresourcelib is not None in the required places.