Instead of repeating hard-coded 'pelicanconf.py' values throughout
Invoke's task.py template, assign default settings file names to
variables, and use those variables where applicable.
The default setting for OUTPUT_PATH is already 'output', so it would be
more DRY to use the existing default value instead of a hardcoded
'output' string.
Removes the `livereload` dependency from `setup.py`.
Updates the `invoke livereload` task by moving the `livereload` import
into the task function since it is now an optional dependency.
Updates the Invoke section of the documentaion with instructions on
using the `livereload` Invoke task.
