UI Support for tag commands: tagfocus, tagignore, tagshow, tagroot, etc

[NHDaly]

Branched from an existing issue:

I think some support for tags would be good. Ability to specify tagfocus and tagignore as an example.

Originally posted by @aalexand in #207 (comment)

It would be really nice to be able to filter this in the UI.
We're using DataDog's continuous profiling tool, which uses tags quite heavily, and it would be nice to be able to filter them in the downloaded profiles.

Similarly, we recently added threadid and task-id to Julialang profiles in JuliaPerf/PProf.jl#40, and it would be great to be able to filter by threadid and task id. (I'm going to add an option to set -tagroot automatically in PProf.jl, but even if it's not set it would still be nice to be able to filter, through the graph view.)

Thanks!

[NHDaly]

It would also be really nice to be able to adjust the -tagroot and -tagleaf in the UI, so that we can easily reorder the "threadid" and "task ID" since sometimes you'd want to group by threads to ask "what tasks were running on this thread" and sometimes you'd want to group by thread ID and you want to know what threads it ran on, and often you don't care about either and you just want to know what your server is doing overall.