Allow config/validate of branch_label in display_defaults

[emmahodcroft]

As raised in the issue #444. We haven't integrated anything about the display_defaults property branch_label (which is available now in auspice) into augur.

This is mostly because we don't currently have any way in augur to generate many kinds of branch labels - only clade and aa. If clade is available, it's shown by default, otherwise nothing is shown.

However, as raised in the issue above, users may want to specify that aa is shown by default, or be able to specify that even though clade is available, nothing should be shown by default.

This PR gives a simple fix just to allow the inclusion of branch_label into v2-style 'config' files, allows these to be validated, allows them to be exported by export v2 and again, allows them to validate.

Currently the only options permitted are aa, clade, and none. (None doesn't really do anything, but if you specify any non-valid entry, auspice simply displays nothing for branch labels. So, this can be made semi-legit by just restricting users to these three options.)

This should be updated to allow pretty much any value (excepting some special characters, probably) when we get around to integrating other ways of labelling branches into augur.

[jameshadfield]

Thanks @emmahodcroft -- I added a commit so that if a default branch label is set in the JSON then we check it is defined on at least one node in the tree rather than forcing it to be aa, clade, or none. (And I allow the special case of setting the default label to none)

This allows valid JSONs to validate, although I realise augur export can't yet produce them!

Also added a test build to check this.

[emmahodcroft]

Great idea @jameshadfield - thank you!