* Fix showing background img with non-empty baseurl
Also switch globally to relative_url for all baseurl prepends,
which is more robust than custom URL mangling
* More site.baseurl -> relative_url conversion
* remove tag _gen plugin and its folder
* remove deprecated tag index layout
* add tag index page
* link tag index page in nav bar, enable tag links by default
* add button spacer in main css
* fix tag links in post view
* fix tag links on index and pagination
* tag link instuctions removed. it's enabled as default
* Add option to use site title as HTML document title
* Add configurable title separator character
* Keep old document title behaviour when not using use-site-title
Tags will display as plain text if a user adds tags to a post. If user
sets 'link-tags: true' in _config.yml, then Jekyll will make a new page
for each tag which lists all posts with given tag (provided the site is
not deployed through GitHub pages)
