Extract metadata: question and answer option labels, translations

[pr130]

Problem:

to generate high quality reports based on KoboToolBox submissions, it might be necessary to:

• have labels / full text of questions (right now, we return the column names set via the Data column name field in each question's settings), for example for plot titles
• have full text of answer options, e.g. "No, not yet" instead of "no_not_yet" etc.
• have translations of both elements in case it's a multilingual survey and the user wants to do reports in multiple languages

Current solution:

asset <- kobo$get(sprintf("assets/%s", survey_id))
content_survey <- asset$content$survey

followed by custom data wrangling (example here: https://gist.github.com/friep/bf091c471b13589214612017a4dcc502 )

Proposed solution:

• provide functions get_question_labels and get_answer_options (better names welcome) attached to Asset or to Kobo (with survey_id as argument)