Look for WP installation in simple subdir configs

Teach WP-CLI about simple subdirectory installations of WP.

If the current directory contains `index.php` and index.php contains
a simple require of wp-blog-header.php (as the default index.php
and simple modifications of it for WP-in-a-subdirectory both do),
look for the WordPress installation based on that information.
Functional test for autodetection of subdir install
Merge pull request #878 from ziz/find-simple-subdirs

Look for WP installation in simple subdir configs