[commands] New `init` command
[commands] New `init` command

Different level of 'init':
1. default
2. empty
3. template

1. level 'default', this will invoke `couchapp.generator.init_basic`
  $ couchapp init dir
    dir/
      .couchapprc
      .couchappignore
      _attachments/
      lists/
      shows/
      updates/
      views/

2. level 'empty'
  $ couchapp init -e dir
    dir/
      .couchapprc
      .couchappignore

3. level 'template', invoke `couchapp.generator.init_template`
  $ couchapp init -t mytemplate dir
    dir/
      .couchapprc
      .couchappignore
      ...
      vendors
      ...

ref #198

couchapp/couchapp,couchapp/couchapp,couchapp/couchapp,couchapp/couchapp