Reconstruct defaulted arg values from docstring

[sizmailov]

Changes:

• Recognize Python literal strings in default arg as "print-safe"
• Add --print-safe-value-reprs=REGEX CLI option to override the print-safe flag of Value (for custom reprs provided via pybind11::arg_v())
• Add --enum-class-locations=REGEX:LOC CLI option to rewrite enum values as valid Python expressions with correct imports. This change introduces new errors since earlier Enum-like representations (e.g. <MyEnum.Value: 1>) were treated as non-printable Values and rendered as ....

Fixes #145