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.
Merge pull request #1669 from albertogg/jekyll-import-arguments

Add table_prefix option for jekyll-importer
Update history to reflect merge of #1669
add test case for variable include tag in a loop
fix include tag: don't store variable value

This fixes the bug reported in #1495 (comments).
Merge pull request #1726 from maul-esel/include-fix

fix variable include tag in a loop
Update history to reflect merge of #1726