Update install_altair()

[ijlyttle]

Possible values of stage are:

• "release", to install the released version on Conda-Forge
• "candidate", to install a release-candidate using PyPi

In the future:

• "github", to install from the Altair GitHub repository

If the "release" version is newer than the "candidate" version, specifying "candidate" will generate a message then install the "release" version.

To support this, create some internal functions:

• install_altair_release()
• install_altair_candidate()

[ijlyttle]

Instead of creating new functions, capture the arguments to install_altar as a list, then modify that list according to stage and method (conda or virtualenv). Finally, call conda_install() or virtualenv_install().

[ijlyttle]

Decided to do things differently: there is a version argument that defaults to getOption("altair.python.version").

You have to declare explicitly if you are using "conda" or "virtualenv".