Merge branch 'dev'

close #46
doc: add document of <Over>(buffer-complete-prev)
Merge branch 'add-doc-buffer-complete-prev'
Merge branch 'dev'
Merge branch 'dev'
Merge branch 'dev'

1. Handle multi {lhs} handling!
2. Support <expr> mappings
Merge branch 'dev'
Merge branch 'dev'
improve .travis.yaml to test latest vim too
Merge branch 'improve-travis' into dev