Add table_prefix option for jekyll-importer

Add table_prefix to use with WordPress and Joomla importers so they can modify the default value.
Use prefix instead of table_prefix.
Fix link to previous section

There's no `assets` section. Linking to `pagination` instead.
Add Emoji for Jekyll to plugin list.
Update history to reflect merge of #1708
Update history to reflect merge of #1707
Link to an org-mode converter for Jekyll
Update history to reflect merge of #1711
Update history to reflect merge of #1669