Add ability to change text justification in a slide

[chapeter]

It'd be awesome to have the ability to shift the text justification from Center to Left. Some slides just end up looking strange when they are center justified...like lists for example.

Even if its a new [item]: # (slide) tag like [item]: # (slide left)...that'd be granular enough.

For now working around this by altering the generated HTML and adding style="text-align: left;" to the slide