Partial implementation of .Bd -centered.

Partial implementation of .Bd -centered.

In groff, .Bd -centered operates in fill mode, which is relatively
hard to implement, while this implementation operates in non-fill
mode so far.  As long as you pay attention that your lines do not
overflow, it works.  To make sure that rendering is the same for
mandoc and groff, it is recommended to insert .br between lines
for now.  This implementation will need improvement later.