Implement Serialization and Deserialization for Verbosity

This commit adds serialization and deserialization support for the
Verbosity type. The verbosity is serialized as an optional log level
that represents the equivalent log level given the number of verbose and
quiet flags. When deserializing, the log level is converted into a
number of verbose and quiet flags, which keeps one of the flags at 0 and
the other at the difference between the log level and the default log
level.

The internal representation in calculations has been changed from i8 to
i16 to prevent overflows when performing calculations using u8 values.

Fixes: clap-rs#88

Author: joshka