Add commands to skip previous steps

[kalekundert]

Proposed CLI:

# Skip first step
sw skip 1
# Skip previous step
sw skip -1
# Skip previous 2 steps
sw skip -2:-2
# Skip all steps:
sw skip 1:-1

A motivating example: I recently was running a gel, and for various reasons the default instructions for preparing the samples (i.e. adding loading buffer, etc.) were not applicable. Some of my protocols provide options to skip steps that aren't always needed, but there was no such option for this step. I thought about adding an option, but this seems like a more general solution to me.

I also like this because stepwise strives to be as flexible as possible. Having a good set of tools for editing protocols on the command line seems like a good way to achieve this.