Add support for custom allocators to boost::locale::conv::utf_to_utf

[Flamefire]

Add new optional parameter such that the use of custom allocators is supported by boost::locale::conv::utf_to_utf.

Based on (stale) PR #231 with

• Fixed formatting
• Add tests
• Allow passing an allocator instance to string overload
• Allow passing allocator instances without passing the method

Closes #231