Loosen dependency on Jekyll to work with newly released version 1.0.0
rename the liquid tag `javascript_asset_tag` to `js_asset_tag`
add the original liquid tag `javascript_asset_tag` to preserve backward compatibility (`javascript_asset_tag` is an alias for `js_asset_tag`)
add the liquid tag `stylesheet_asset_tag` for enhanced code symmetry (`stylesheet_asset_tag` is an alias for `css_asset_tag`)
update readme to suggest using `js_asset_tag` instead of `javascript_asset_tag`
incorporate `#tag_full_name` in `LiquidBlockExtensions` module