Add lexer support for string extractors with wildcards

[andyscott]

Extractors against interpolated strings containing wildcards are currently unsupported and cause a ScalaLexerException with message "invalid string interpolation".

Snippet that triggers an exception:

def unwrapAnonInstance(tree: Tree) = tree match {
  case Block(
    q"""$_ class $name1 extends ..$_ { ..$stats }""" :: Nil,
    q"new ${ Ident(name2) }"
    ) if name1 == name2 ⇒
    Some(stats)
  case _ ⇒
    None
}

This PR fixes this. (Note: I wasn't positive what token should be emitted, so I left it at USCORE)

[kiritsuku]

test this please

[ghprb-bot]

Refer to this link for build results (access rights to CI server needed):
https://jenkins.scala-ide.org:8496/jenkins//job/ghprb-scalariform-sbt/6/

[kiritsuku]

LGTM