Added VimuxRunCommandInDir + doc

As I was working with vimux, I really wanted an option to execute a specified command in the folder of the currently opened file (current buffer).

That's what VimuxRunCommandInDir does. I added my two personal use cases to the documentation.

Please tell me what you think.
Won't this leave the terminal in the new directory if the command exits non-zero?

Could it instead be run in a subshell like this?
`call VimuxRunCommand("(cd ".shellescape(expand('%:p:h'), 1)." && ".a:command." ".l:file.")") `