Add question and selection of optional container/conda cache dirs

[jfy133]

This is to close #20

[jfy133]

Current issues:

• How to get both the variable AND the path to print as options
• create_cachedir_options() just adds the var name to a list if it's true, so need to maybe replace check_var_in_env to export a dict with var, path pair
• How to return output of create_container_scope to pass to writing, as now exporting two sets of options (container engine itself + options; and now cache dir)

[jfy133]

Also to add: if manual selection made, check directory exists