sources: Refactoring json cache and fix regexp
sources: Refactoring json cache and fix regexp

- Separate to get_cache (Thanks @tweekmonster!)

- Fix regexp for among the string

Signed-off-by: Koichi Shiraishi <13fbd79c3d390e5d6585a21e11ff5ec1970cff0c@zchee.io>

zchee/deoplete-go,zchee/deoplete-go