improvement: implement interface for all prompts under Ask module

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
feature: added checkbox selection

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
restyle checkbox to utf8 symbols

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
bugfix: select checkbox items based on space keypress

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
add checkbox to Ask module

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
add checkbox example

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
return an array with selected items in checkbox

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
added checkbox example to readme

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>