Support regex for expected_command verification

[aalsabag]

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Description of issue or feature request:
I have read through some of the previous issues regarding this matter #46
I understand the idea behind only warning when a user command differs from an expected command. I am not proposing that we change this. All that I am proposing is adding a new optional parameter verify_command_alignment called regex that takes a string. If the regex is passed, we use that to perform a regex match. If they do not match, we print out a warning as we normally do.

I have already written this out in a branch and have written some unit tests to illustrate this.

I can change how we pass in the regex, but for now I thought to make expected_command optional as well as regex optional. I'm trying to thing of a better approach.

I'll add the documentation if you guys are ok with this. For now the PR is a draft
#611