Allow plugins to add assets

[marvinchin]

At the moment, plugins do not have the ability to add custom assets to be served. Plugins authors are hence unable to use custom assets for their plugins (e.g. CSS stylesheets, images).

Plugin authors should be able to add custom assets to be served from as part of the MarkBind site. This allows MarkBind plugins to be more versatile.

[acjh]

Related: #700