added useInvalidElements to xml_options_t

This setting can be used to have dc2uddf spit out elements that are not
strictly a part of uddf. Most parsers *should* ignore them, but it makes
the document no longer validate. In exchange for this schema brokenness
you get a document with <event> tags about your dive that right now
dc2uddf doesn't currently handle.

Signed-off-by: Patrick Wagstrom <patrick@wagstrom.net>
Added --invalid option for additional debugging

When given the --invalid option dc2uddf will include <event> and
<vendor> tags. While these are not part of the uddf schema, they're
really helpful for understanding what is going on with your dive
computer.

Also, I don't handle every type of event - mainly because my dive
computer doesn't support every type of event and I don't have data
to test some of the events. For example, I'm not going to
intentionally blow a deco stop just to see what my computer reports.

Signed-off-by: Patrick Wagstrom <patrick@wagstrom.net>