init pxt-modules with init command

[pelikhan]

• [x] don't override existing mkc.json
• [x] always drop pxt-modules with init command
• [x] removed --pxt-modules from build command

Usage: cli init [options] [editor]

initializes the project, optionally for a particular editor

Options:
  --colors               force color output
  --no-colors            disable color output
  --debug                enable debug output from PXT
  --symlink-pxt-modules  symlink files in pxt_modules/* for auto-completion
  --link-pxt-modules     write pxt_modules/* adhering to 'links' field in mkc.json (for pxt cli build)  
  -h, --help             display help for command