Add extra parameters to HTML tag and convert data tags for bootstrap_…

[ralokt]

…button and bootstrap_label

Implements #376

I made a possibly controversial decision when I implemented this - parameters that start with "data_" get their underscores converted to hyphens. This is because data attribute names must start with the string "data-", but anything that fits that is not a valid identifier and can't be passed from Django templates.

On the other hand, blind conversion might be a bad idea? It would, however, be more consistent.

In any case, making it consistent is a matter of simplifying the code, so I'm just going to leave it that way, and am open to changing it.

[coveralls]

Coverage increased (+0.2%) to 85.146% when pulling 8ed9bf8 on ralokt:master into e80c739 on dyve:master.

[dyve]

Thanks. This would be a precedent for other functions, and I think it's too big to do in the final release(s) / stage(s) of this package.