Add --root-directory option as alternative to .shellspec file

[pdesjardins90]

My development workflow requires that I pass all arguments as command line options, so my .shellspec file is empty. I can't however delete it because of the limitation of shellspec not having another way to detect the project's root.

I propose adding the --root-directory option to specify it from the command line, what do you think?

[ko1nksm]

Is it unacceptable to add an empty .shellspec file?

[pdesjardins90]

Not unacceptable, but not optimal either

Please consider this request as a simple enhancement, not a life or death feature

This is just the first time I see a library requiring a file supposed to contain only optional stuff