0.6.0 - ninja by default

• ninja is now the default make tool: you will now need ninja installed on
  linux and mac systems to perform a build (it has always been listed in the
  installation instructions), build output will look different (but builds
  will be substantially faster).
• new yotta outdated subcommand
• --save option is now default to yotta install (an installed module will be added to module.json). The --save flag is now ignored.
• support mbed login in addition to github login
  where applicable
• display search results from all configured registries
• fix crash in yt init when running with defaults
• improve handling of github errors
• various other minor bug fixes and improvements

Thanks to @theotherjimmy and @BlackstoneEngineering for contributing many of the improvements!