Add watch-content-base option

[charlespwd]

Please check if the PR fulfills these requirements

• An example has been added or updated in examples/ (for features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce?

• Feature

What is the current behavior? (You can also link to an open issue here)
Currently, if a file changes under the content-base, nothing happens. #350

What is the new behavior?
With the --watch-content-base option, file changes in the content-base will trigger a full reload.

Does this PR introduce a breaking change?

• No

Fixes #350. If/when this gets merged. We should update the official docs.

[SpaceK33z]

Very nice PR!

I noticed the watcher isn't closed, so if you'd use the Node.js API, dev-server wouldn't stop properly. See Server.prototype.close.

[charlespwd]

Thanks!

That's a very good point. Should be corrected now. Let me know if you want me to squash commits before you merge.

[SpaceK33z]

Cool addition, thanks again :). Could you document this in the new webpack v2 docs? See devServer options.

[charlespwd]

Sure thing!