A few small modifications to address some issues.
- Closes Issue #18: CMakeLists.txt allows disabling SIMD instruction for compilations targeting ARM and other platforms.
- Closes Issue #21: Added some text in README.md to clarify the need to use an escape character before { and }.
- Closes Issue #22: Added some text in README.md to clarify how the downsampling option -downsamp should be used for the case of 422 (Obviously more work on documentation is needed).
- Closes Issue #23: It looks like some older versions of ffprobe are sensitive to my ppm header format, even though it is totally valid; this release modifies this header to make more acceptable. I think the new format should be compatible with more applications.