Allow multiple database directories to be specified

[yeban]

Through the config file and through the command-line interface.

e.g.,

sequenceserver --database_dir /db1,/db2,/db3
sequenceserver -d /db1 -d /db2

This came up on the mailing list: https://groups.google.com/d/msg/sequenceserver/7qxsNTmOI3A/WOS2yGaRAQAJ

[GeneDesign]

Would love to see this feature! Thank you for opening the issue.