fix export of strandedness

[rneher]

The strand of translated features is represented in the exported json via + or -. But this export didn't work as intended because augur translate wrongly assumed feature_location.strand is boolean when instead it is +1 or -1. This commit additionally accounts for unknown (0 -> ?) or unspecified strandedness (None -> None).

The line changed was suggested by @tsibley in this comment

superseeds PR 966

[rneher]

It looks as if our tests were happily asserting that the wrong annotation is correct.

$ python3 "../../scripts/diff_jsons.py" translate/data/tb/aa_muts.json $TMP/aa_muts.json
-  {}
+  {'values_changed': {"root['annotations']['*MtCM']['strand']": {'new_value': '-', 'old_value': '+'}, "root['annotations']['PE_PGRS39']['strand']": {'new_value': '-', 'old_value': '+'}, "root['annotations']['PPE13']['strand']": {'new_value': '-', 'old_value': '+'}, "root['annotations']['Rv0010c']['strand']": {'new_value': '-', 'old_value': '+'}, "root['annotations']['Rv0011c']['strand']": {'new_value': '-', 'old_value': '+'}, "root['annotations']['Rv0039c']['strand']": {'new_value': '-', 'old_value': '+'}, "root['annotations']['Rv0323c']['strand']": {'new_value': '-', 'old_value': '+'}, "root['annotations']
[...]

The gene *MtCM for example should be on the negative strand:

MTB_anc	RefSeq	gene	2134273	2134872	.	-	.	ID=gene1921;Dbxref=GeneID:885772;Name=*MtCM_Rv1885c;gbkey=Gene;gene=*MtCM;

[huddlej]

Beyond the merge conflict, this looks good to me!