API for creating StatTrackView on bed files

[iromeo]

At the moment we can use BigFiles, both bigwig signal and bigBed intervals. Let's support not only bigBed files, but also ordinary bed files. May be we can convert & cache bed files to bigBeds.

E.g. attached screenshot show 2 stats views, first for raw bigWig signal, the second for called peaks.

I think may be useful when working with called peaks from peak callers