Request json_parser::read_json(boost::filesystem::path const&, ...)

There are currently two signatures available:
- `read_json(std::string const&, ...)`
- `read_json(std::basic_istream<...> const&, ...)`

In case of Windows and a file name with non-standard characters I cannot rely on `read_json(std::string const&, ...)`.

I therefore use this workaround:
`boost::filesystem::path const filename(L"Some foreign language file name");`
`boost::filesystem::ifstream ifs(filename);`
`read_json(ifs, ...);`

In case of an error, the exception thrown does not contain the filename and I have to construct a message of my own.

I would like to request these additions:
- `read_json(boost::filesystem::path const&, ...)`
- `read_json(std::wstring const&, ...)`

Start with the existing read_json(std::string const&, ...) and read_json(std::basic_istream<...> const&, ...) entry points, then trace how filenames and errors are handled on Windows. Done means the requested boost::filesystem::path and std::wstring overloads exist and errors retain the filename for non-standard character paths.

Written by the indexing model from the issue text.