bed3 parsing was broken in older versions of rustbio. Now resolved with
latest version.

Change hint on bed format to indicate correct bed4.

• Added --html flag which writes interactive vplot heatmaps in html format (named as: prefix.vplot.html).
• Autonamed vplot matrix csv files are now suffixed with .vmatrix.csv instead of .csv.

add hint about indexing bam file if missing.

* The 0.3 series of `vplot` introduces a breaking change for matrix
formatting. Matrices are now returned such that large fragments are at
the top. This can be reverted using the `-i` flag to invert the matrix
to have small fragments at the top instead (pre-0.3 behavior).

* `-o` flag for writing to a file path.

* the `--multi` flag writes matrices for each region in the input bed
file. An optional prefix or ouput directory can be added using the `-o`
flag.