Extend strong-cli with generator-loopback and loopback-sdk-angular-cli

/to @sam-github 
/cc @altsang 
/cc @ritch 

The PR addresses the following issues:

1. Add generator-loopback and loopback-sdk-angular-cli as peer dependencies so that they can be installed with strong-cli and use `slc update` for upgrading other dependencies.

2. Add a message for `slc lb` command

3. Upgrade to loopback workspace 2.6.0 for loopback 1.x.