Add stairs plot

[sethaxen]

This PR adapts @jkrumbiegel's implementation of a stairs plot in https://gist.github.com/jkrumbiegel/529936a3a6f158715343601d9487880b.

Example:

f = Figure()
x = 1:10
y = 11:20
f[1, 1] = Axis(f; title="step=:pre")
stairs!(f[1, 1], x, y)
f[2, 1] = Axis(f; title="step=:post")
stairs!(f[2, 1], x, y; step=:post)
f[3, 1] = Axis(f; title="step=:center")
stairs!(f[3, 1], x, y; step=:center)
f

[sethaxen]

Additional options we could add:

• ability to style vertical and horizontal segments differently
• ability to provide a marker and marker style options to simultaneously scatter

[lazarusA]

this other case will also be so useful... (histstep)

https://gist.github.com/jkrumbiegel/5baa32d0c010cb842e037603a18663cd

[sethaxen]

Probably the simplest thing to do for histstep would be to factor the histogram creation code in hist into its own helper function, and then the difference between hist and histstep is only the attributes and whether it calls barplot! or stairs! internally.