Option refactoring

Went ahead and did what was discussed in issue #8.

There's now a few new options:
- `php_sql_heredoc` - indicate whether highlight SQL in heredocs.  Defaults true.
- `php_html_in_heredoc` - indicate whether to highlight HTML in heredocs.  Defaults true.
- `php_html_load` - indicate whether to load HTML syntax file at all.  If false, the HTML syntax file isn't loaded and no HTML syntax highlighting will occur.  This will force `php_html_in_strings` and `php_html_in_heredoc` to false and turn off highlighting outside of `<?php ?>` tags.  Defaults true.

All of the options have defaults that produce the current behavior so people who update won't see a change unless they want to.

I noticed a couple of options didn't have descriptions in the docs, so I added some based on what I guessed they did.

I also consolidated the folding option explanations as I felt they were a little confusing being split apart.
Merge looks good! I have a comment on this line, should it not go under `php_html_in_heredoc`?