Modular printing

[MarcMush]

following #357

this is a draft of what could look like modularity

defining ProgressElements
the default printing (similar to current) is
("Progress: ", ProgressPercentage(), " ", ProgressBar(), " ", ProgressETA(), ", ", ProgressSpeed())

This is absolutely not ready for merging, it only applies to Progress, doesn't have adaptative width, I've removed some code, and didn't test much

I'd like some feedback to see if that's the direction we want to go

[MarcMush]

this is how to use it at the moment:

using ProgressMeter
using ProgressMeter: ProgressElements,ProgressPercentage,ProgressBar,ProgressETA,ProgressSpeed

elements = ProgressElements(("Computing...", ProgressPercentage(), " ", ProgressBar(length=30)," ", ProgressETA(), ", ", ProgressSpeed()))

p = Progress(20; elements)

for i in 1:20
    sleep(0.1)
    next!(p)
end