If the environment is not set through command line argument or
environment variable, the bundle loads either 1) the only environment,
2) the only environment with the default flag set.
While working on artifact upload and workspace interrogation I realized
this mutator interface needs to:
1. Operate at the whole bundle level so it can apply to both
configuration and internal state
2. Include a `context.Context` parameter for a) long running operations
and b) progress reporting
Previous interface:
```
Apply(*config.Root) ([]Mutator, error)
```
New interface:
```
Apply(context.Context, *Bundle) ([]Mutator, error)
```
Used to inspect the bundle configuration after loading and merging all
files.
Once we add variable interpolation this command could show the result
after interpolation as well.
Each of the mutations to this configuration is observable, so we could
add a mode that writes each of the intermediate versions to disk for
even more fine grained introspection.
Pieter Noordhuis
shreyas-goenka