Add webpack-cli to dev dependencies

[SeanKilleen]

Contributes to #389 (I think).

Not sure of the best way to build / test / verify this, but happy to do so with a point in the right direction.

[SeanKilleen]

Closing in favor of more discussion on #389.

[akrantz]

To test it:

1. In a command window at the generator-office directory, build and link:

c:\generator-office> npm run build
c:\generator-office> npm run link

2. go to another empty directory where you can create a new project.
   cd c:\test
3. link to generator-office package
   npm link generator-office
4. run Yo Office and choose the project template to create.
   yo office
   Choose React, TypeScript, Outlook. Type project name, e.g. "React TypeScript Outlook Add-in".
   cd "React TypeScript Outlook Add-in"
5. build and run to verify the add-in works.

[akrantz]

After you do this, you can do "npm uninstall -g generator-office" and then "npm install -g generator-office" to reinstall the latest released package.