Allow user to install additional packages in the current environment

[pierrepo]

We sometimes need to install additional packages in a running environment, either because we forgot to install these packages in the environment image, either because we want to perform a new analysis that requires additional dependencies.

One solution would be to make the conda binary (and probably the base env) visible. This is related to issue plasmabio/tljh-repo2docker#39
Since the environment is running in a temporary Docker container, new packages installed with conda in the base environment will be ephemeral (and need to be reinstalled each time the user load the environment)

[pierrepo]

What's your opinion on this @jtpio?