Fix hidden placeholder content override

When a user sets a data placeholder, they really want a placeholder.

Set content to !important and ensure it's not overridden (e.g. by Bootstrap).

Closes #255

Signed-off-by: Nick Semenkovich <semenko@alum.mit.edu>
Merge pull request #256 from semenko/master

Fix hidden placeholder content override
Merge pull request #1 from daviferreira/master

#252, #158