Allow setting the default workspaces directory

The default workspaces directory default_workspaces_path determines, where Axii will try to create a new workspace in the wizard. By default, this is the home directory ~:

$ axii config --global default_workspaces_path
~

When running axii w create, Axii will thus propose to create the workspace code (default workspace name) into ~/code:

$ axii w create
...
Specify the workspace.
Workspace directory: ~/code

This default location can be adjusted with the new config variable default_workspaces_path. For example, if you have your workspacess on a dedicated hard drive mounted on /code, run:

$ axii config --global default_workspaces_path /code
Set 'default_workspaces_path' (global) to /code.