Add the :execute method to activate! and withenv

[frankier]

This PR adds an experimental method kw arg to :execute and withenv. In this case, the Conda environment will be activated in a bash subshell, meaning all the activation hooks will be run. All environment variables updated or created in this subshell are then changed in the current process. This method requires bash to be available, and has only been tested on Linux.

I've found this useful for executing hook scripts such as the one provided by Stan.

I've not written tests yet, but can do if you confirm this is something you would like to merge.