Fix capa optioninput template to honor inline flag

The "inline" flag is used to allow capa input elements to be displayed inline.  This is useful, for example, when questions are written as paragraph text, with inputs embedded within the text.  

The `textline` input element honors the inline flag -- see https://github.com/edx/edx-platform/blob/master/common/lib/capa/capa/templates/textline.html#L1

This PR fixes the `optioninput` template to also honor the inline flag (note that https://github.com/edx/edx-platform/blob/master/common/lib/capa/capa/inputtypes.py#L328 already provides the inline flag in the mako context).