[BuildStream] bst show all and bst build all flags

[Phillip Smyth <phillip smyth codethink co uk>]

Hi all,

I'm close to introducing an effective "build world" command and a 
"validate world" command.
This is designed to build and validate all elements in a project, 
regardless of dependencies.

There is however the issue that a `bst build --all` command already 
exists which does something different.

So, i wanted to reach out to everyone to get feedback on how we should 
name these flags.

So far as i understand it, the options are:
both use `bst [command] --all`
both use `bst [command] --world`
or both use  `bst [command]`

The third option defaulting a command without a named element, to the 
equivalent to --world.

I am open to new ideas/options and hopefully we'll have a final decision 
soon so i can get everything finished

best regards,
Phillip Smyth [nexus]