Add ability to append buttons or content without an add-on span

[anark]

Adds the option to append things like buttons that do not work when they are within an span.add-on

[sodabrew]

Thanks! This looks interesting. Does it match with existing formtastic behavior? I would prefer not to create too much special / new behavior that only exists in formtastic-bootstrap.

[anark]

Formtastic has no concept of appended or prepended inputs as these are a bootstrap concept.

This pull request adds the 'append_content' and 'prepend_content' options to go along with the existing 'append' and 'prepend' options.

[anark]

An alternative would be to alter the existing 'append' and 'prepend' options to not include the '<span class="add-on"'>. That way it could be manually passed or excluded at the users discression. However this would not be backwards compatible as existing usages of the append option would lose the 'add-on' span wrapper

[sodabrew]

Sounds good, I'll take it!