Add formatting option for setting number of spaces after # in a comment

[rcjsuen]

From my experience, Dockerfiles usually have one space after the # in a comment. However, this could theoretically be different for whatever reasons.

#no space comment
# comment
#  also a comment
#   of course this one too

We should provide an option to define the number of spaces after a # for a comment. This would turn the above into the following...

# no space comment
# comment
# also a comment
# of course this one too