Improved default gitignore file

Hey @wesleytodd, thanks for YeoPress. Very useful stuff. :)

I thought I'd improve the default .gitignore file a bit—generally, you never want to version-control configuration files (will differ on every system, and contains sensitive information like database pass, so I added wp-config.php. As well, ignoring logfiles and automatically generated sitemap.xml-files (like generated by Yoast et al.) can't really do any harm.

Up to you whether you want to prescribe these things to YP by default, of course, but in my humble opinion, they're sensible defaults.