add :target_blank option

Twitter text js has a targetBlank option. To make it
easier to go back and forth between twitter text js
and twitter text ruby add the target_blank option to
match targetBlank from js. Ensure that this target_blank
differs from url_target by adding "target='_blank'" to
all auto_linked entities.
add :target_blank option to list of available options
add test for new :target_blank option

Ensure that all types of links are auto linked AND
target="_blank" is added too all these links.
Merge pull request #105 from eileencodes/add_target_blank_option_to_auto_link

Add target blank option to auto link