Add a "shell" command similar to the existing "global" and "local"
commands, which sets the version in an environment variable instead of
writing it to a file. This was inspired by the similar functionality in
rbenv.
It works by adding a wrapper function for the asdf command. It forwards
to a "sh-shell" command that returns the exports as shell code which is
then evaled by the wrapper. This is a little gross, but we need to run
the code in the shell context in order to set variables.
Resolves#378
* Rename `asdf which` -> `asdf current`
* Output `set by $path` with current command
* Use dummy plugin in current_command test
* Hide "set by" message if derived from legacy file
