Use dash-plugin protocol by default

[KabbAmine]

It may be interesting to use dash-plugin:// protocol by default and execute zeal's binary only if the option g:zv_zeal_executable is not empty (For the ones using a portable version for example).

The only benefit from using the url protocol is that zeavim will be compatible with dash in mac and velocity in windows.

I let the question in suspend so if anyone is interested, let me know.