Option refactoring

[swekaj]

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.

[StanAngeloff]

Thanks for your work on this pull request! :+1: