check for new version on execution

[forkwhilefork]

When the tool is executed, we should check if there is a new version. If there is, we should tell the user and offer to fetch it.

I think we can achieve this by simply doing a git fetch if we haven't done one in the last minute, and checking if the current commit is the most recent.

I think ignoring the branch is okay - that gives the user the option to use a feature branch, or at some point we might have separate develop and stable branches.