Provide a command to link executables

[frostming]

Brought here by your talk on PyConTW. I am taking your suggestions and found this great project.

Can you add a new command link(add perhaps unlink, too) to link the executables under PythonUp/versions/x.x/bin to PythonUp/bin?

[uranusjr]

There is a command use that you can use like this

pythonup use 3.6
# Links things in versions/3.6/bin to bin

pythonup use 3.6 3.5
# Links things in versions/3.6/bin and version/3.5/bin to bin
# The former takes precedence if there are name conflicts.

pythonup use --reset
# Removes things in bin

[frostming]

Oh, greate