emsdk

Improve activation message (#562)

* Improve activation message

The activation message is now IMHO more informative and
actionable:

```
$ emsdk activate latest
Setting the following tools as active:
   node-12.18.1-64bit
   releases-upstream-e7e39da9c81faecd9ecf44065cee864d76e4e34d-64bit

Next steps:
- To conveniently access emsdk tools from the command line,
  consider adding the following directories to your PATH:
    $HOME/wasm/emsdk
    $HOME/wasm/emsdk/node/12.18.1_64bit/bin
    $HOME/wasm/emsdk/upstream/emscripten
- This can be done for the current shell by running:
    source "$HOME/wasm/emsdk/emsdk_env.sh"
- Configure emsdk in your bash profile by running:
    echo 'source "$HOME/wasm/emsdk/emsdk_env.sh"' >> $HOME/.bash_profile
```

* .