Let user specify the column for a node

Some nodes can appear correctly in one of several columns.

Imagine a source node for a node in column 4 -- it could correctly appear in column 1, 2, or 3.

Currently (June 2016), such a node is forced into column 1.
After implementing #11, un-checking the 'Justify' option will put such a node into column 3 (one before its target). But there is still no way for the user to say that they want it in column 2.

Add & implement a syntax for specifying a column for a node. Straw example:

:nodename #069.3 nodelevel=2
(My first instinct is col=2 or column=2 instead, but since diagrams may eventually have a vertical-orientation option, 'column' will be confusing at that point. Other possible labels: level, group, nodegroup...)

There must be some error-checking involved here as well. If the user specifies an invalid nodegroup (say, 4 or 5 or 6 in the current example), we should toss out the user's value and handle it in the default manner (1 or 3 here, depending on the justification setting).

Ideally we can then give the user a helpful warning message about their invalid input as well, though that is less crucial.

nowthis / sankeymatic

Let user specify the column for a node #23