Adds ability to set selectedView when setting a custom view

[almcintyre]

Changes proposed in this pull request:

• Adds optional selectedView when adding a custom view for a Menu Item

Fixes issue #347

[cody1024d]

I could really use something like this as well. I think it could be implemented a bit differently, to avoid attaching multiple views, but this feature in general is really important. (I would simply create a SelectableCustomView protocol, that customView has to implement, which would expose an isSelected variable, or something, that would be used to trigger changes within the custom view itself)

@kitasuke Any chance something like this could make it into the project?