Add option to allow setting default slide alignment classes

[pippinbarr]

Currently the default alignment for a slide is 'left top', but it would be helpful to be able to specify this in the options to avoid having to set every single slide to a new default (e.g. I'm currently using 'left middle' on every slide).

Would be great to have this as an option in the constructor, e.g.

var slideshow = remark.create({
    defaultSlideAlignment: 'left middle'
});

[abelards]

I can suggest a few workarounds:

• put your body with class middle
• put some CSS on remark-slide-container

I think it's best if we stay as close to HTML/CSS as we can, so people learn just that, instead of adding options to remark, which forces us to test and maintain, and our users to learn one more thing.

What do you think?

[pippinbarr]

Totally fair, and makes a lot of sense to outsource to CSS in general. Guess the only tradeoff is that users who aren't comfortable in CSS might struggle to "see" that solution, but depends really on how fundamental you view certain options versus others, so it's totally legit to offload this way I think!