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

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 https://github.com/google/pprof/issues/207#issuecomment-324711178_

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 https://github.com/JuliaPerf/PProf.jl/pull/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!