Make Viper.Sub() inherit values from the bound pflags of the parent

[atanasovskib]

Sub creates a new instance from a sub-tree, but bound flags
are not passed down to the new instance. This commit adds
the bound flags that are prefixed as the provided k to the
new instance.

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[bogdandrutu]

In the same notes #871. I think we need to inherit all configs/options except the ".config" member.