Add defaultContainerWidth option for SSR rendering support

[itoldya]

Since react-grid-gallery relies on clientWidth which is available only on the client side it is not possible to render component on the server side - it always returns a wrapper without thumbnails. I added optional prop defaultContainerWidth to workaround it.

I also reformat tables in the readme. My IDE highlighted an error For compatibility reasons all table rows should have borders (pipe symbols) at the start and at the end so I decided to apply a fix.

[benhowell]

Thanks @itoldya for the SSR width prop.
I'll merge.

Is it possible to get a hint from the client side to automate the setting? I'm not too familiar with SSR (or react post version 15) so I'm pretty clueless as to what can be gathered from the client side to help automate the prop.

Thanks!

[benhowell]

Thanks for updating the readme btw ;-)