Support for concurrent uses of different terragrunt versions

[AlainODea]

Right now tgswitch changes the symlink to a terragrunt binary on the PATH or via -b.

If I invoke a script which internally uses terragrunt multiple times from different directories concurrent with different .terragrunt-version files I will get (somewhat) non-deterministic behaviour.

If tgswitch provided a single terragrunt command that exec'd the version in the nearest .tgswitchrc or .terragrunt-version up the directory tree, it could support arbitrary numbers of concurrent sessions with varying versions.