Support for plotting BED tracks, other minor updates

Added --bed-tracks <BED file> to add tracks below plot, 1 track per
label in the label column of the BED file.

Also a minor update to allow changing colors of each marker arbitrarily
by specifying a column in the metal file containing a color name or hex
code (e.g. 'navyblue' or '#FF00AA'.) Use colorCol='name_of_col'.