Add `showzero` option to always show before first iteration

[IanButterworth]

I have a very long running first iteration (compilation) and would like to know it has started without having to add another print.

Originally came up here #141 (comment)

[miles-1]

Such an argument would be really helpful, especially for nested processes, e.g.:

using Progress

p1 = Progress(10, desc="first ", color=:green)
for i in 1:10
    p2 = Progress(10, desc="second", color=:blue, offset=1)
    for j in 1:10 sleep(0.1); next!(p2) end
    finish!(p2)
    next!(p1)
end
finish!(p1)

It doesn't make a lot of sense to have the inner processes' progress bar show up before the outer ones' does.

Looks like this would be fixed with this.

Edit: in case it is helpful to others, I found that using update!(p1, 0, force=true) will help give the desired behavior.