Proposal: .iamy-version file

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