Replace deprecated pseudo-private API usage

[alerque]

The pseudo-private _VimuxOption function has been deprecated and an
official public VimuxOption method is available. However the way
defaults are handled is also changed. If you don't want to use the
default value, just don't. Hard coding the prompt here seems to make
more sense that even bothering to ask what the option value is.

[michalwski]

@spiegela any chance of getting this PR merged? Looks like it can help with currently not working VimixPromptCommand.

The error I'm getting with newest vimix plugin is:

Error detected while processing function VimixPromptCommand:
line    1:
E117: Unknown function: _VimuxOption
E116: Invalid arguments for function input
E15: Invalid expression: input(_VimuxOption("g:VimixPromptString", "Mix Command? "))
line    2:
E121: Undefined variable: l:command
E116: Invalid arguments for function <SNR>57_VimixRunCommand