Minor bug fixes and improvements

- Resolve symlinks in path to plug.vim (#67)
- `syntax enable` instead of `syntax on`
- Refactor PlugUpgrade (#72)
- Remove plugin directory on failure during installation (#75)
- Add `U` keybinding for updating plugins (#79)
- Use blackhole register when deleting lines
- Expand argument to plug#begin (#82)
- Suppress error messages from `which` command
- Improve &runtimepath mgmt (#85, #88)
- Enable syntax only during startup
