Support TAP14's SubtestDocument

[bowerscd]

TAP 14 supports SubtestDocument, which effectively is the request for #14 -
currently tappy will impose an error on any documents adhering TAP14 to with this new SubtestDocument:

 (<file=/tmp/tmpgw_n99ao>, '') Only one plan line is permitted per file.

It's trivially workaround-able, however (iterate through failures, remove the one with the exact short description "Only one plan line is permitted per file.").

[mblayman]

Yep, my policy was to make tappy compliant with the TAP spec and no more. TAP 14 just got merged (I was one of the contributors), so I haven't invested any time into it yet.

There's a lot of work here to make this work properly. Thanks for submitting the issue.