Two New Options: applyHeightAttributes and applyCenterAttributes

[madmanlear]

These updates are very exclusively for email development, specifically more awful clients like Outlook 2013.

• applyHeightAttributes does exactly what applyWidthAttributes does, but with height!
• applyCenterAttributes with check to see if margin: auto (or some variant with auto) is applied. If the element is an image, it's wrapped in a <center> element. If it's a table, the attribute align="center" is added.

[jonkemp]

@madmanlear are there any open issues which this pull request is addressing?

[madmanlear]

@jonkemp There aren't, though I'm happy to open some. These are just very common needs for email development that I kept running into.

[jonkemp]

@madmanlear if you don't mind, that would be helpful. Even if it's just one.

[madmanlear]

@jonkemp done!

[jonkemp]

@madmanlear thanks. Can you actually split this into two pull requests, one for each feature? Tests would be good as well.