archetype SampleMain: extend brooklyn.cli.Main
Adds docs for archetype

- and placeholder page for YAML blueprints
Merge branch 'refactor/archetype-to-use-airline-for-cli' of https://github.com/aledsage/brooklyn into review-cli
fix new SampleMain in archetype (by removing unwanted template string ${package});
make some more changes to superclass Main so subclassing and extending is simpler,
and so help messages are more appropriate and catalog items are loaded;
and tweak the (already excellent!) documentation --
note our convention is to include the `$` in bash scripts, as these are filtered out when copied.
don't verify URL during init, so that unit tests work without internet
Merge pull request #1207 from ahgittin/review-cli

builds on #1200, fixing and tidying a few small things