Add select command

[interstateone]

This PR adds a new select command, which uses xcode-select under the hood. The motivation for adding this feature is that for users who are installing multiple Xcode versions simultaneously, and so xcodes is valuable, switching between those versions is a likely need and not easy to do with xcode-select alone. I've been using a small shell script for many years to solve exactly this problem, but it would be nice to have this inside xcodes instead.

It can be used like this:

# Interactively select an installed Xcode

$ xcodes select
Available Xcode versions:
1) 10.1
2) 10.2.1
3) 11.0
4) 11.2
5) 11.2.1
6) 11.3
7) 11.4 (Selected)
Enter the number of the Xcode to select: 6
xcodes requires superuser privileges to select an Xcode
macOS User Password: 
Selected /Applications/Xcode-11.3.0.app/Contents/Developer

# Select an Xcode by providing a version

$ xcodes select 11.4.0
Selected /Applications/Xcode-11.4.0.app/Contents/Developer

# Select an Xcode by providing a path

$ xcodes select /Applications/Xcode-11.4.0.app
Selected /Applications/Xcode-11.4.0.app/Contents/Developer

# Print the currently selected Xcode path

$ xcodes select -p
/Applications/Xcode-11.4.0.app/Contents/Developer

The output of the xcodes list and xcodes installed commands also annotate the version that's selected in the same way.

If you uninstall the currently selected Xcode version, xcodes will automatically select the latest installed version, if there is one, so that the selected path is a valid one.

Closes #47
Closes #82

[civatrix]

Looks great. Love all the tests!