Add server feature
Implemented server feature(experimental)
skk server does not support okuri.
Improve error
Improve for server timeout
Support for error result
Portnum to port
Improve dictionary initialization
Add documentation
Add type in g:eskk#server
Update example
`string[-1]` is always empty string (#148)
remove debug code (#148)
warn if g:eskk#server is defined but can't use it (#148)
trivial doc-fix (#148)