New BED colouring option for scores that are pvalues

[iromeo]

For tracks with pvalues - the less value, the better, so most confident peaks will be transparent + we interested in -log(pvalue). Let's support:

• -log(x), where if 'x' is zero, we will replace it with minimal absolute 'x_i' value

otherwise we have to always update the track and add some log columns into it.

[iromeo]

E.g. could be used for https://github.com/JetBrains-Research/jbr/issues/251 feature & coloring that makes best pvalues visible, not transparent