Support `@implicit ...` keyword

[mkurz]

Now that we support @var, @val, @lazy val it makes sense IMHO to also support

• @implicit var
• @implicit val
• @implicit lazy val
• @implicit foo (def called foo)

instead of requiring values and variables to be prefixed with like @val implicitSomeVar (like we do now).

Should not break anything since implicit is a scala keyword and could not be used in twirl until now anyway.