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
Previously, if we ran a command like `asdf install` and we encountered a
plugin that didn't exist, asdf produces an error saying "No such
plugin". Without knowing which plugin it could be referring too, we'd
have to manually go through each plugin in `.tool-versions` to find the
culprit.
With this commit, we'll now also include the plugin name as part of the
messaging for easier debugging.
Instead of just looking in $HOME/.tool-versions, allow the default
tool-versions file location to be specified through the variable
$ASDF_TOOL_VERSIONS.
