Proposal: .iamy-version file

[mipearson]

We've had a few issues where people have gotten stuck/confused because the version of iamy they're running is newer/older than the version last used in that repository.

I'd like to add an .iamy-version file to be written to working directory that is in part inspired by Ruby bundler's version checking.

Behaviour:

• if it doesn't exist, it's created as .iamy-version with the current output of iamy --version and an INFO message is printed
• if it does exist and your version is newer than what's in .iamy-version, the file is overwritten and an INFO message is printed
• if it does exist and your version is the same, nothing happens
• if it does exist and your version is older by minor or patch version, a WARN message is printed
• if it does exist and your version is older by major version, an ERROR message is printed and execution stops