Automatic check for available updates

[tlmii]

It is currently pretty easy for a user to install the tool once and continue using that installed version without realizing there are updates.

We should investigate implementing a check to see if there is an update. This could be as simple as an API call to nuget to check for the most recent version, or maybe we could somehow use the new dotnet tool search functionality.

Some general thoughts on implementation:

• Probably on by default, with an opt out pref
• Probably only check for non-preview versions, but with a pref to check for preview
• We should probably keep it simple and just tell them there's an update and give them the command to issue - not try to auto update or anything like that
• Probably want to skip the check if they pass run or help arguments at the command line to avoid it showing up in automated usage.