Enable syntax only during startup

Related discussion can be found in #83. If the user choose not to use
syntax highlighting, he can disable it by putting `syntax off` after
`plug#end()`. But since `plug#end()` is called by `PlugInstall` or
`PlugUpdate`, it is reenabled on those commands which is not desirable.
This commit fixes the issue by changing `plug#end` to do `syntax enable`
only during Vim startup.