Tool window to select default `--config` flag value from values defined in `.bazelrc`

[hypdeb]

This is a feature request / idea.

As documented here, the .bazelrc file allows defining aliases for collections of command lines arguments to pass to the Bazel commands, like this:

<COMMAND>:<ALIAS> <ARGUMENT>

I think it would be valuable to offer the user an easy way to switch between configurations for each command (or maybe also globally?) in a UI element in the editor. I got the idea from looking at #292.

@jfirebaugh what do you think?