Add a reset function

[ryanlovett]

Proposed change

We recommend that people manually move aside their working directories if their state somehow inhibits syncing with the upstream repo. It'd be nice if nbgitpuller could have a reset argument which could perform this step automatically. It is similar in spirit to JupyterLab's workspace reset feature.

Alternative options

People can manually login and rename their working directories from their notebook apps or a terminal.

Who would use this feature?

People who need a quick "click link to fix" option.

(Optional): Suggest a solution

Consider renaming the old working directory by adding a timestamp to the name. This ensures it has a unique name in case the directory is reset more than once.