General attribute support

[wasade]

Support for a dict of attributes in BP would be nice. It may make sense to revisit having attribute information associated with the opening parenthesis of a node, and instead of a lookup table. For instance:

t = '(()())'
my_attributes = ['foo', 'bar', 'baz']
lookup = [0, 1, 1, 2, 2, 0]  # index in t -> attribute index

Possible precursor to #8 as it leverages having BP.open available from the first pass of topology parsing, however, constructing the attribute lookup at time of parse should be possible and obviate BP.open.