add parameter to use with flask opentracing

[qianmoke]

when flask_tracer is true, use flask_tracer's current span as default span. this helps us tracing easy when using sqlalchemy with flask.

[carlosalberto]

Hey @qianmoke

This won't be needed as part of the OT 2.0 version (https://github.com/opentracing/opentracing-python/tree/v2.0.0) This will provide active Span management and we won't require to save extra stuff around for detecting the parent Span.