improve vendor command

[chris-rock]

This PR makes improves the usability of the inspec vendor command.

Case 1:

$ cd profile
$ inspec vendor
# This creates a ./vendor and ./inspec.lock

Case 2:

$ inspec vendor path/to/profile
# This creates a path/to/profile/vendor and path/to/profile/inspec.lock

[arlimus]

Thank you @chris-rock !

[chris-rock]

This implements the vendoring of profiles into a profile directory as defined in #1283