Allow to close the overlay

[mgol]

The DOM-based error overlay is a nice feature but sometimes I need to fire up code with linting errors just for a moment and the overlay blocks access to the page. Up until now I was just refreshing the page in such a case but after PR #132 was landed it stopped being possible.

How about a Close button in a corner that would close the overlay when clicked? The overlay should return on the next HMR update.

[glenjamin]

My personal approach to this is to not have linting be part of the webpack build, I only run it on the cli, in my editor, or on the CI server.

Another option would be to have the lint checks at "warning" level, as the overlay doesn't appear for warnings (they're only reported in the console).

That said, having a way to dismiss the overlay seems reasonable to me, so I'd take a PR which added a close button.

[mgol]

OK, I'll dive into it tomorrow then.