Add 'list' script message for querying existed keyword

[Youxikong]

Introduce a script message for quering existed keyword. I think it would be helpful if keywords of third party scripts are complicated like state=(mute and 'checked').

[tsl0922]

current implementation of keyword_to_menu does not work on multiple instances of a keyword, there should be a new design for get / set too.

[zhongfly]

small typo in commit message: `list' should be 'list'

[Youxikong]

I have another idea: make keyword_to_menu[keyword] a table, and pass a 'id' when send menu-get-reply message.
close to #51