ensure default output path is absolute

[rossipedia]

Example of problem:

$ webpack-cli init

Insecure about some of the questions?

https://github.com/webpack/webpack-cli/blob/master/INIT.md

? Will you be creating multiple bundles? No
? Which module will be the first to enter the application? ./src
? Which folder will your generated bundles be in? [default: dist]:
? Are you going to use this in production? No
? Will you be using ES2015? No
? Will you use one of the below CSS solutions? No
? If you want to bundle your CSS files, what will you name the bundle? (press enter to skip)
? Name your 'webpack.[name].js?' [default: 'config']:

... (npm output) ...

Congratulations! Your new webpack configuration file has been created!

$ webpack
Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
 - configuration.output.path: The provided value "./dist" is not an absolute path!

$ cat -n webpack.config.js
...
    15          output: {
    16                  filename: '[name].bundle.js',
    17                  path: './dist'   <-- RUH-ROH
    18          },
...

This PR converts the line above to:

path: path.resolve(__dirname, 'dist')

[jsf-clabot]

All committers have signed the CLA.

[evenstensberg]

LGTM! Thanks! 😻

[evenstensberg]

Signing @okonet for second review, this is okay for me.