Add support for "obstacle" key

[grinapo]

Planning a meaningful bike route (but probably hiking as well) would greatly gain from obstacle=* tags, especially obstacle=vegetation (which is route-wise equivalent to smoothness=impassable or visibility=no with an arbitrary softening).

See wiki or taginfo.

Thanks.

ps:

assign misc_penalty = ...
 ...
 add if obstacle=vegetation|fallen_tree then 6 else 0

[afischerdev]

Added obstacle key in #458
But line and lock are not added, they have already seamark tags.