v2.7.0 Image (LUTS) & text extraction improvements

What's Changed

• Integrate samples from pdf-samples repository into this repo by @PrinsFrank in #266
• Update CONTRIBUTING.md with instructions on adding samples by @PrinsFrank in #267
• Add sample from #255 by @PrinsFrank in #268
• Add samples from previous issues by @PrinsFrank in #269
• Fix colorspace parsing issues by @PrinsFrank in #270
• Add support for LUTs in rasterized images by @PrinsFrank in #274
• Add sample from #235 by @PrinsFrank in #271
• Only fall back to identity decoding when toUnicodeCMap is not set, fixes #254 by @PrinsFrank in #276

Full Changelog: v2.6.3...v2.7.0