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.
