Support CommonMarkSettings via MarkdownViewEngineOptions

[hishamco]

@jskeet this piece that required to expose CommonMarkSettings via MarkdownViewEngineOptions, but i'm not sure if there 's an elegant way to support this via @Page directive

Perhaps some of the settings such as RenderSoftLineBreaksAsLineBreaks will be easy to implement, because there 's no CSharp parser yet that may simplify the process, so I think this suitable for app settings for at the moment

[hishamco]

Addresses issue #3

[jskeet]

Great, thanks - will try it out tonight :)

[hishamco]

I'm still wonder if we can expose the entire CommonMarkSettings object or shall we create a new MarkdownSettings object with the commonly used properties, and use CommonMarkSettings behind the scene