Add new option for controlling whether to display categories on menu or not

Conflicts:
	docs/changelog.rst
	pelican/tests/output/basic/tag/bar.html
	pelican/tests/output/basic/tag/baz.html
	pelican/tests/output/basic/tag/foo.html
	pelican/tests/output/custom/tag/bar.html
	pelican/tests/output/custom/tag/baz.html
	pelican/tests/output/custom/tag/foo.html
Merge pull request #578 from davidjb/display-categories-option

Add option to control displaying categories on menu