Add extrude support

[d1g]

It makes sense to enable this mode when editing objects with building=*, but not natural=water (and such).

https://josm.openstreetmap.de/wiki/Help/Action/Extrude
Not everything must be implemented: "ctrl" mode (as default way to move way segments) and probably "basic mode"

It makes no sense to draw arbitrary shaped object, then use Q tool, then correct some portion of the object, then press Q again.

It is time-consuming to human to draw 90-degree objects using pattern mentioned above.

[bhousel]

Yes I would also like the ability to select segments and extrude them.

Both extrusion (selecting a segment and moving it), and polygon operations (#435), are useful tools described here: https://www.mapbox.com/blog/fast-building-tracing/

[RedAuburn]

I'd really love to be able to extrude, it's the main thing keeping me mapping buildings in JOSM.
Extruding speeds up rectangular building creation so much, I feel it'd be a very valuable feature to add to iD ^-^