feat: adds a --watch flag to only showing stats that contain a specific statement

[bernardotorres]

My personal use case was that I was profiling a service.
The main thread would be reading from Kombu all the time, so the interface was mostly showing operations from the main thread and not from the request threads, which were the ones I was interested in.

[coveralls]

Coverage decreased (-0.5%) to 56.52% when pulling 8763a5d on bernardotorres:master into f149119 on what-studio:master.

[coveralls]

Coverage decreased (-0.5%) to 56.52% when pulling 8763a5d on bernardotorres:master into f149119 on what-studio:master.

[sublee]

Thanks for sending a great feature.

But I worry about a performance at is_watched which checks the give statistics recursively. This function would be called for every live statistics update into a viewer. I guess that when we use the live profiling for a complex program, the delay of is_watched could indicate some perceivable UI lag.

Can we find a better approach?