Add transports field to Onionoo's details documents

Bridges announce their pluggable transports in their extra-info descriptors. We scrub most of that information in the sanitizing process, but at least the transport names survive this process:

@type bridge-extra-info 1.2
extra-info ec2bridgercf30a0b4 7C86C0E30E2E267BB508B7B78A375D740E72B07D
[...]
dirreq-v3-tunneled-dl complete=0,timeout=0,running=0
transport obfs2
bridge-stats-end 2013-04-07 06:43:24 (86400 s)
[...]

Onionoo could add a new field transports to include transport names in its details documents. That would allow bridge operators to at least check whether their bridge announces the configured transports.