Change HTML links to open a new tab

All of the visualizations linked in the description for each type of
data visualization will open a new tab, instead of changing the current
context.

References to other resources, like the UniFrac paper, Greengenes etc,
were also changed to have the same behaviour.

Test module for format is updated to reflect these changes.

This should fix issue #91
Merge pull request #95 from ElDeveloper/issue_91

Change HTML links to open a new tab