View template attributes

[Michiel-s]

Just like proposed in #1084 this issue request for possibility to specify template attribute for user defined templates for leaf nodes.

Example of current syntax

VIEW "PersonEmail"    : Person { email : personEmail } HTML TEMPLATE "PersonEmail.html" ENDVIEW

INTERFACE "Active projects" : V[SESSION*Project];projectActive BOX
    [ "Projects"      : I[Project]
    , "Projectleider" : pl <PersonEmail>
    ]

Requested attribute syntax, e.g.

INTERFACE "Active projects" : V[SESSION*Project];projectActive BOX
    [ "Projects"      : I[Project]
    , "Projectleider" : pl <PersonEmail attrWithoutValue attrWithValue="test">
    ]

Like with the box attributes #1084, these attributes must be available in the template engine (haskell part) and in the generated view.json for the prototype framework.