Flag common misuse of output functions

[nickdaugherty]

The return value of functions that output should generally not be used. Things like:

echo bloginfo( 'name' );
echo the_title();
echo wp_title();
echo esc_html( printf( ... ) );
echo esc_url( the_permalink() );

[sboisvert]

Might be more efficient to figure out the return value with static analysis and anything that returns void or not a string maybe should be flagged.