Allow optionally specifying branch lengths for trees

[kollienne]

This treats any pair that begins with a number (2, "a") as a branch-length and child node, allowing arbitrary branch lengths in trees.

I'm open to feedback on this, as it is it probably breaks existing documents where trees are labelled using integers.

[johannes-wolf]

Thank you! Can you please add a test case that shows this feature?

[johannes-wolf]

Closed, see #649.