fix for VimuxTogglePane, check for valid shell on _VimuxNearestIndex(), others

[pantosaur]

• the VimuxTogglePane function was not working properly, e.g. -s and -t flags were interchanged.

• Changed _VimuxNearestIndex to make sure vimux does not use a pane hosting a pts for which the process running is not in the list g:VimuxShells, (default ["bash","zsh"]). This makes sure we don't send random keys to a random program open in the nearest pane. If the runner type is "window", a pane will only be valid if it is the only pane of the window, and the window is not the same as the one of the vim program.

• g:VimuxRunnerIndex is changed to a list that looks like [window,pane] and functions that take the runner index are given the joined string, which works in all cases regardless of "window" or "pane" runner type.

[alerque]

Thanks for taking the time to prepare this contribution. It looks to me like some of these issue have been since addressed other ways, and certainly the code based has changed enough that it introduces conflicts for other reasons too. There may still be some of this that is useful though, but I'm having trouble putting my finger on what. Any chance you are around to look into it and clarify/rebase as needed?