Currently only one fill_between graph can be added to the plot.
This patch aims to add the possibility of having an array of possible
fill_between.

Example:
Currently the fill_between is as:
args = {}
args["fill_between"] = dict(y1=df[High],y2=df[Low],color=red...)
fig, axes = mpf.plot(.....,**args)

This patch also add the possibility of:
args = {}
args["fill_between"] = [
  dict(y1=df1[High],y2=df1[Low],color=red...),
  dict(y1=df2[High],y2=df2[Low],color=green...)
]