Make behavior of unknown tags configurable

[veger]

Unknown tags are treated as text, unless configuration is used to instead raise an exception.

Fixes #33

[coveralls]

Coverage increased (+0.01%) to 99.862% when pulling dff2b48 on ignore-unknown-tags into f96cb1c on master.

[almaron]

A much neater solution. But I'd rather have an option to completely ignore those tags (i.e. not render anything) or treat them as text.

[almaron]

So maybe we can set the configuration to be slightly more flexible.
Like have :exception, :text, :ignore possibilities.

[veger]

you mean you want to hide the tags themselves and only show text in between?
Could be a neat option yes, let me see if we can change the code and grab this from existing tag_info