Add hyphenize_underscores option.

Most CSS and javascript (for example, twitter bootstrap) uses hyphens, not underscores, in class names and IDs. This change makes it possible to write something like div.nav_bar and have the class end up as nav-bar not nav_bar. For compatibility with erector 0.9.0, this is an option rather than unconditional, but the patch also changes the documentation to say that the default for the option is likely to change to hyphens in the future.