FeatureRequest: Add option to disable external cmdline

[zhmars]

Would it be possible to add an option that would allow to disable the external cmdline ? E.g.:let g:gonvim_draw_extcmdline = 0.

I don't need it and it maybe disturbing sometimes.

[Shougo]

The feature seems from below code.

https://github.com/dzhou121/gonvim/blob/cbaa76e46df467f27d1175be17a915bbdbd3b2ac/editor/editor.go#L176

Related issue: https://github.com/dzhou121/gonvim/issues/77