Bug 18665 - Translatability: Add tt filter to allow html tags inside tt directives

Bug 18665 - Translatability: Add tt filter to allow html tags inside tt directives

HTML tags inside template toolkit directives are not allowed because of translation issues.
Add a filter that handles HTML tags.

Note you need to write quotes ' around the text you want displayed as a
heading

To test:
- Apply patch
- Add [% USE HtmlTags %] to the top of a tt file
- Add something like [% 'My nice title' | $HtmlTags tag="h1" %] to the tt file
- Verify that in output 'My nice title' has h1 tags
- Change template directive to something like
  [% 'My nice title' | $HtmlTags tag="h1" attributes='title="This is a nice title attribute"' %]
- Verify that title attribute displays in output (source code or tooltip on 'My nice title')

Notes: - Tests are planned for a second patch
       - Update for Wiki coding guidelines

Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>