Adding contexts

[malcolmholmes]

"Contexts" are a major improvement to Grizzly's usability. Now, instead of having to set (and reset) environment variables, they can be set once, and users can easily switch between them.

As well as storing credentials, contexts can also store other settings, such as:

• only-spec: Tells Grizzly only to store the raw file content without the envelope
• output-format: Allows switching between YAML and JSON when writing files

This issue is intended to group together these tasks and usability improvements.