This appeared to be the least messy approach to hack in support for
IsSet also checking environment variables to see if a particular
cli.Flag was set without making backwards incompatible changes to the
interface.
I intend to fix this more properly in v2, probably by adding another
method to the cli.Flag interface to push the responsibility down as it
occurred to me that it was really the `Flag`s themselves that offer
support for configuration via the environment as opposed to the
`context` or other supporting structures. This opens the door for the
anything implementing the `Flag` interface to have additional sources of
input while still supporting `context.IsSet`.
This change allows a context value to be set after parsing. The use case is updating default settings in a Before func.
An example usage:
```
f, err := os.Open(configPath)
if err == nil {
config, err := docli.NewConfig(f)
if err != nil {
panic(err)
}
c.Set("token", config.APIKey)
}
```
Compared to BoolFlag type, BoolTFlag treats 'true' as the default value
for the flag.
Without it, we have to use --no-action flag if we set the action is done
in default. But sometimes it is bad to maintain flags with negative meanings.
And it will be painful if we change the default value for the flag.
As this implementation, it keeps all existing functionality. So it
is compatible with old versions.
