Support for a FollowTheSignage instruction

[daniel-j-h]

There are situations for which following a sign is the proper hint for users.

For example: the noref=yes sign which represents an exit without further details.

http://taginfo.openstreetmap.org/tags/noref=yes#wiki

We should think of adding a instruction for these situations to the api in v6.

(Another use-case would be: complex situations we just can't handle. Acknowledge this and let make the user aware of it)

@pratikyadav @MoKob