Add "m17nlib-use-candidate-window?" option to the m17nlib module.

* scm/m17nlib-custom.scm
  - Use "m17nlib-use-candidate-window?" option.
* scm/m17nlib.scm
  - Define "m17nlib-use-candidate-window?" option
    based on the patch by icy in #25.
Add a section for 1.8.6.

* NEWS
  - (Overview of changes from 1.8.5 to 1.8.6): New section.