[RFC] Provide thumbnails and previews for *.six / *.sixel

[saitoha]

My working directory quickly fills with files like test1.six and test-yyyymmdd.six, so I made it easier to preview their contents in explorer and Finder.

On develop branch, WIC decoder(Windows) and Quicklook Extension(macOS) are provided.

To install WIC decoder (Windows):

On Mingw64 environment:

$ ./configure --enable-wiccodec --enable-register-dll && make && make install

To install Quicklook Extension (macOS):

$ ./configure --enable-quicklook-extension && make && sudo make install

After installation, consider logging out and back in once; Please report any issues.