[plugin kkk] Add option for kkk

Add plugin option "multiSequence". If this option is `true`, prevent key
events when only multi sequence key-binds. However, priority order of
preventing is "sites" > "multiSequence". By the way, this option is
`false` by default.
[plugin kkk] refactoring
Merge pull request #148 from senda-akiha/add-opt-for-kkk

[plugin kkk] Add option for kkk