Introduce concept of current profile

[ycombinator]

With #301 elastic-package got an awesomely powerful system of user-supplied configuration profiles, managed via the elastic-package profiles sub-command suite. As a follow up enhancement, it would be nice if users could select or set the current profile.

Specific impacts of adding the concept of a current profile:

• expose a command like elastic-package profiles set <profile ID> or similar that would allow users to set the current profile. Initially, before this command is ever run by a user, the profile with the lowest creation timestamp, which will usually be the default profile should be the current profile.
• profile-aware commands like elastic-package stack should use the current profile as the default profile, when a different profile isn't explicitly supplied to such commands.
• the elastic-package profiles list command should mark which profile in the list is the current profile.
• the elastic-package profiles delete <profile ID> should forbid deletion of the current profile.