Improve option parsing facility so that option applicability can be specified

[oniony]

Currently the option parsing facility cannot capture option relationships which means invalid option combinations are not detected generally: instead these have to be checked for explicitly by each command.

There needs to be a way to specifiy:

• An option that is only valid in the presence of other specific options.
• An option that is not valid in the presence of other specific options.

[oniony]

Issue #65 would potentially address this.