Add BoolTFlag type

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.
Merge pull request #70 from unihorn/27

Add BoolTFlag type
fix(context): copy StringSlice Flag correctly
Merge pull request #73 from unihorn/43

fix(context): copy StringSlice Flag correctly
sync with upstream.